One single place for all the iPhone firmwares of all generation models starting from iOS 3.1.2 as the lowest firmware possible. iOS 1.0 & 2.0 are outdated and we don't encourage you to stay on those versions. the links provided below are stock/original firmwares and comes direct from Apple servers. Please do not ask for torrents, Google it yourself. Please note that restoring older firmware versions truly depends on available SHSH blobs saved for your device. Read the SHSH blobs Doc if you don't know what those are. This Doc will be updated whenever Apple release new version.
NOTE : IPSW links of older iOS versions are removed from doc. We will remove links of firmware that are no longer being signed by Apple. We will add them back if a new downgrade method is found.
iOS 3.1.2 (released on 5th October 2009)
iPhone 2G : http://appldnld.apple.com.edgesuite...91008.32pNe/iPhone1,1_3.1.2_7D11_Restore.ipsw
iPhone 3G : http://appldnld.apple.com.edgesuite...91008.Xsd32/iPhone1,2_3.1.2_7D11_Restore.ipsw
iOS 3.1.3 (released on 22nd January 2010)
iPhone 2G : http://appldnld.apple.com.edgesuite...00202.4orot/iPhone1,1_3.1.3_7E18_Restore.ipsw
iPhone 3G : http://appldnld.apple.com.edgesuite...00202.pbnrt/iPhone1,2_3.1.3_7E18_Restore.ipsw
iOS 4.1 (released on 3rd September 2010)
iPhone 3G : http://appldnld.apple.com/iPhone4/061-7932.20100908.3fgt5/iPhone1,2_4.1_8B117_Restore.ipsw
iPhone 3GS : http://appldnld.apple.com/iPhone4/061-7938.20100908.F3rCk/iPhone2,1_4.1_8B117_Restore.ipsw
iOS 4.2.1 (released on 21st November 2010)
iPhone 3G : http://appldnld.apple.com/iPhone4/061-9853.20101122.Vfgt5/iPhone1,2_4.2.1_8C148_Restore.ipsw
iOS 6.1.6 (released on 21st February 2014) (for iPhone 3GS only)
iPhone 3GS : http://appldnld.apple.com/iOS6.1/091-3457.20140221.Btt3e/iPhone2,1_6.1.6_10B500_Restore.ipsw
iOS 7.1.2 (released on 30th June 2014)
iPhone 4 : http://appldnld.apple.com/iOS7.1/031-4812.20140627.cq6y8/iPhone3,1_7.1.2_11D257_Restore.ipsw
iPhone 4 (8GB Model) : http://appldnld.apple.com/iOS7.1/031-4785.20140627.zZ42j/iPhone3,2_7.1.2_11D257_Restore.ipsw
iOS 9.3.5 (released on 25th August 2016)
iPhone 4S : http://appldnld.apple.com/iOS9.3.5/...3834D2D062/iPhone4,1_9.3.5_13G36_Restore.ipsw
iOS 10.3.3 (released on 19th July 2017)
iPhone 5 (GSM) (Model No. A1428) : http://appldnld.apple.com/ios10.3.3...E3/iPhone_4.0_32bit_10.3.3_14G60_Restore.ipsw
iPhone 5 (GSM+CDMA) (Model No. A1429) : http://appldnld.apple.com/ios10.3.3...E3/iPhone_4.0_32bit_10.3.3_14G60_Restore.ipsw
iPhone 5c (GSM) : http://appldnld.apple.com/ios10.3.3...E3/iPhone_4.0_32bit_10.3.3_14G60_Restore.ipsw
iPhone 5c (GSM+CDMA) : http://appldnld.apple.com/ios10.3.3...E3/iPhone_4.0_32bit_10.3.3_14G60_Restore.ipsw
iOS 11.2 (released on 2nd December 2017)
iPhone 5s (GSM) (Model No. A1533) : http://appldnld.apple.com/ios11.2/0...03F/iPhone_4.0_64bit_11.2_15C114_Restore.ipsw
iPhone 5s (GSM+CDMA) (Model No. A1530) : http://appldnld.apple.com/ios11.2/0...03F/iPhone_4.0_64bit_11.2_15C114_Restore.ipsw
iPhone 6 Plus : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone_5.5_11.2_15C114_Restore.ipsw
iPhone 6 : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone_4.7_11.2_15C114_Restore.ipsw
iPhone 6s : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone_4.7_11.2_15C114_Restore.ipsw
iPhone 6s Plus : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone_5.5_11.2_15C114_Restore.ipsw
iPhone SE : http://appldnld.apple.com/ios11.2/0...03F/iPhone_4.0_64bit_11.2_15C114_Restore.ipsw
iPhone 7 : http://appldnld.apple.com/ios11.2/0...F/iPhone_4.7_P3_11.0_11.2_15C114_Restore.ipsw
iPhone 7 Plus : http://appldnld.apple.com/ios11.2/0...F/iPhone_5.5_P3_11.0_11.2_15C114_Restore.ipsw
iPhone 8 (GSM+CDMA) : http://appldnld.apple.com/ios11.2/0...F/iPhone_4.7_P3_11.0_11.2_15C114_Restore.ipsw
iPhone 8 Plus (GSM+CDMA) : http://appldnld.apple.com/ios11.2/0...F/iPhone_5.5_P3_11.0_11.2_15C114_Restore.ipsw
iPhone X : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone10,6_11.2_15C114_Restore.ipsw
iOS 11.2.1 (released on 13th December 2017) (CURRENT VERSION)
iPhone 5s (GSM) (Model No. A1533) : http://appldnld.apple.com/ios11.2.1...2/iPhone_4.0_64bit_11.2.1_15C153_Restore.ipsw
iPhone 5s (GSM+CDMA) (Model No. A1530) : http://appldnld.apple.com/ios11.2.1...2/iPhone_4.0_64bit_11.2.1_15C153_Restore.ipsw
iPhone 6 Plus : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone_5.5_11.2.1_15C153_Restore.ipsw
iPhone 6 : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone_4.7_11.2.1_15C153_Restore.ipsw
iPhone 6s : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone_4.7_11.2.1_15C153_Restore.ipsw
iPhone 6s Plus : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone_5.5_11.2.1_15C153_Restore.ipsw
iPhone SE : http://appldnld.apple.com/ios11.2.1...2/iPhone_4.0_64bit_11.2.1_15C153_Restore.ipsw
iPhone 7 : http://appldnld.apple.com/ios11.2.1...iPhone_4.7_P3_11.0_11.2.1_15C153_Restore.ipsw
iPhone 7 Plus : http://appldnld.apple.com/ios11.2.1...iPhone_5.5_P3_11.0_11.2.1_15C153_Restore.ipsw
iPhone 8 (GSM+CDMA) : http://appldnld.apple.com/ios11.2.1...iPhone_4.7_P3_11.0_11.2.1_15C153_Restore.ipsw
iPhone 8 Plus (GSM+CDMA) : http://appldnld.apple.com/ios11.2.1...iPhone_5.5_P3_11.0_11.2.1_15C153_Restore.ipsw
iPhone X : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone10,6_11.2.1_15C153_Restore.ipsw

NOTE : IPSW links of older iOS versions are removed from doc. We will remove links of firmware that are no longer being signed by Apple. We will add them back if a new downgrade method is found.
iOS 3.1.2 (released on 5th October 2009)
iPhone 2G : http://appldnld.apple.com.edgesuite...91008.32pNe/iPhone1,1_3.1.2_7D11_Restore.ipsw
iPhone 3G : http://appldnld.apple.com.edgesuite...91008.Xsd32/iPhone1,2_3.1.2_7D11_Restore.ipsw
iOS 3.1.3 (released on 22nd January 2010)
iPhone 2G : http://appldnld.apple.com.edgesuite...00202.4orot/iPhone1,1_3.1.3_7E18_Restore.ipsw
iPhone 3G : http://appldnld.apple.com.edgesuite...00202.pbnrt/iPhone1,2_3.1.3_7E18_Restore.ipsw
iOS 4.1 (released on 3rd September 2010)
iPhone 3G : http://appldnld.apple.com/iPhone4/061-7932.20100908.3fgt5/iPhone1,2_4.1_8B117_Restore.ipsw
iPhone 3GS : http://appldnld.apple.com/iPhone4/061-7938.20100908.F3rCk/iPhone2,1_4.1_8B117_Restore.ipsw
iOS 4.2.1 (released on 21st November 2010)
iPhone 3G : http://appldnld.apple.com/iPhone4/061-9853.20101122.Vfgt5/iPhone1,2_4.2.1_8C148_Restore.ipsw
iOS 6.1.6 (released on 21st February 2014) (for iPhone 3GS only)
iPhone 3GS : http://appldnld.apple.com/iOS6.1/091-3457.20140221.Btt3e/iPhone2,1_6.1.6_10B500_Restore.ipsw
iOS 7.1.2 (released on 30th June 2014)
iPhone 4 : http://appldnld.apple.com/iOS7.1/031-4812.20140627.cq6y8/iPhone3,1_7.1.2_11D257_Restore.ipsw
iPhone 4 (8GB Model) : http://appldnld.apple.com/iOS7.1/031-4785.20140627.zZ42j/iPhone3,2_7.1.2_11D257_Restore.ipsw
iOS 9.3.5 (released on 25th August 2016)
iPhone 4S : http://appldnld.apple.com/iOS9.3.5/...3834D2D062/iPhone4,1_9.3.5_13G36_Restore.ipsw
iOS 10.3.3 (released on 19th July 2017)
iPhone 5 (GSM) (Model No. A1428) : http://appldnld.apple.com/ios10.3.3...E3/iPhone_4.0_32bit_10.3.3_14G60_Restore.ipsw
iPhone 5 (GSM+CDMA) (Model No. A1429) : http://appldnld.apple.com/ios10.3.3...E3/iPhone_4.0_32bit_10.3.3_14G60_Restore.ipsw
iPhone 5c (GSM) : http://appldnld.apple.com/ios10.3.3...E3/iPhone_4.0_32bit_10.3.3_14G60_Restore.ipsw
iPhone 5c (GSM+CDMA) : http://appldnld.apple.com/ios10.3.3...E3/iPhone_4.0_32bit_10.3.3_14G60_Restore.ipsw
iOS 11.2 (released on 2nd December 2017)
iPhone 5s (GSM) (Model No. A1533) : http://appldnld.apple.com/ios11.2/0...03F/iPhone_4.0_64bit_11.2_15C114_Restore.ipsw
iPhone 5s (GSM+CDMA) (Model No. A1530) : http://appldnld.apple.com/ios11.2/0...03F/iPhone_4.0_64bit_11.2_15C114_Restore.ipsw
iPhone 6 Plus : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone_5.5_11.2_15C114_Restore.ipsw
iPhone 6 : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone_4.7_11.2_15C114_Restore.ipsw
iPhone 6s : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone_4.7_11.2_15C114_Restore.ipsw
iPhone 6s Plus : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone_5.5_11.2_15C114_Restore.ipsw
iPhone SE : http://appldnld.apple.com/ios11.2/0...03F/iPhone_4.0_64bit_11.2_15C114_Restore.ipsw
iPhone 7 : http://appldnld.apple.com/ios11.2/0...F/iPhone_4.7_P3_11.0_11.2_15C114_Restore.ipsw
iPhone 7 Plus : http://appldnld.apple.com/ios11.2/0...F/iPhone_5.5_P3_11.0_11.2_15C114_Restore.ipsw
iPhone 8 (GSM+CDMA) : http://appldnld.apple.com/ios11.2/0...F/iPhone_4.7_P3_11.0_11.2_15C114_Restore.ipsw
iPhone 8 Plus (GSM+CDMA) : http://appldnld.apple.com/ios11.2/0...F/iPhone_5.5_P3_11.0_11.2_15C114_Restore.ipsw
iPhone X : http://appldnld.apple.com/ios11.2/0...9DC24E03F/iPhone10,6_11.2_15C114_Restore.ipsw
iOS 11.2.1 (released on 13th December 2017) (CURRENT VERSION)
iPhone 5s (GSM) (Model No. A1533) : http://appldnld.apple.com/ios11.2.1...2/iPhone_4.0_64bit_11.2.1_15C153_Restore.ipsw
iPhone 5s (GSM+CDMA) (Model No. A1530) : http://appldnld.apple.com/ios11.2.1...2/iPhone_4.0_64bit_11.2.1_15C153_Restore.ipsw
iPhone 6 Plus : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone_5.5_11.2.1_15C153_Restore.ipsw
iPhone 6 : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone_4.7_11.2.1_15C153_Restore.ipsw
iPhone 6s : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone_4.7_11.2.1_15C153_Restore.ipsw
iPhone 6s Plus : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone_5.5_11.2.1_15C153_Restore.ipsw
iPhone SE : http://appldnld.apple.com/ios11.2.1...2/iPhone_4.0_64bit_11.2.1_15C153_Restore.ipsw
iPhone 7 : http://appldnld.apple.com/ios11.2.1...iPhone_4.7_P3_11.0_11.2.1_15C153_Restore.ipsw
iPhone 7 Plus : http://appldnld.apple.com/ios11.2.1...iPhone_5.5_P3_11.0_11.2.1_15C153_Restore.ipsw
iPhone 8 (GSM+CDMA) : http://appldnld.apple.com/ios11.2.1...iPhone_4.7_P3_11.0_11.2.1_15C153_Restore.ipsw
iPhone 8 Plus (GSM+CDMA) : http://appldnld.apple.com/ios11.2.1...iPhone_5.5_P3_11.0_11.2.1_15C153_Restore.ipsw
iPhone X : http://appldnld.apple.com/ios11.2.1...B2783B2/iPhone10,6_11.2.1_15C153_Restore.ipsw