Future

• Testable FTP support will be removed
• Breaking Testable Insecure HTTP will be deprecated
• Breaking RFC 2047 support for Content-Disposition header will be removed

Firefox 80 Nightly

• Pragma header will be ignored if Cache-Control is specified
• Breaking DTLS 1.0 support in WebRTC has been removed

Firefox 79 Beta / Developer Edition

Firefox 78 ESR

• X-Content-Type-Options:nosniff is now enforced even if Content-Type is not given
• Breaking Testable TLS 1.0/1.1 support has been disabled by default
• image/webp has been added to default HTTP Accept header
• HTTP Public Key Pinning is no longer supported
• Breaking X-Content-Type-Options: nosniff now applies to top-level documents, causing some pages to be downloaded
• X-Frame-Options: Allow-From directive has been removed
• TLS 1.0 and 1.1 are now deprecated, disabled in Nightly
• All FTP resources are now downloaded instead of being rendered
• Referer header size is now limited to 4 KB
• UA string no longer reveals 32-bit Firefox running on 64-bit OS
• Resolving Promise returned by fetch() is now deferred during page load

Firefox 78

• Breaking Testable TLS 1.0/1.1 support has been disabled by default

Firefox 77

Firefox 76

Firefox 75

• X-Content-Type-Options:nosniff is now enforced even if Content-Type is not given

Firefox 74

Firefox 73

Firefox 72

• image/webp has been added to default HTTP Accept header
• HTTP Public Key Pinning is no longer supported
• Breaking X-Content-Type-Options: nosniff now applies to top-level documents, causing some pages to be downloaded

Firefox 71

• TLS 1.0 and 1.1 are now deprecated, disabled in Nightly

Firefox 70

• X-Frame-Options: Allow-From directive has been removed
• All FTP resources are now downloaded instead of being rendered
• Referer header size is now limited to 4 KB

Firefox 69

• UA string no longer reveals 32-bit Firefox running on 64-bit OS
• Resolving Promise returned by fetch() is now deferred during page load

Firefox 68 ESR

• XHR load, loadend, readystatechange events are now deferred during page load
• Response's default statusText has become empty string
• Breaking setTimeout()/setInterval() are now deferred during page load
• Loading unknown protocol no longer raises exception
• Remote JAR support has been completely removed

Firefox 68

• XHR load, loadend, readystatechange events are now deferred during page load

Firefox 67

• Response's default statusText has become empty string

Firefox 66

• Breaking setTimeout()/setInterval() are now deferred during page load

Firefox 65

Firefox 64

• Loading unknown protocol no longer raises exception

Firefox 63

Firefox 62

Firefox 61

• Remote JAR support has been completely removed

Firefox 60 ESR

• Breaking Basic auth credentials are now encoded in UTF-8 instead of ISO-8859-1
• HTTP auth dialog can no longer be triggered by cross-origin images
• Breaking Remote JAR support has been disabled again
• Breaking URLs with no host name are now treated as invalid
• NPN support has been removed

Firefox 60

Firefox 59

• Breaking Basic auth credentials are now encoded in UTF-8 instead of ISO-8859-1
• HTTP auth dialog can no longer be triggered by cross-origin images

Firefox 58

Firefox 57

Firefox 56

Firefox 55

• Breaking Remote JAR support has been disabled again

Firefox 54

• Breaking URLs with no host name are now treated as invalid

Firefox 53

• NPN support has been removed

Firefox 52 ESR

• Regressed Predictive prefetch makes duplicate HTTP requests in iframes, causing false ad impressions
• Regressed NTLM authentication may fail against HTTPS site
• Breaking Increased HPACK maximum table size breaks specific servers
• Accept header for XHR has been simplified
• SPDY/3.1 has been disabled
• Accept header for images has been simplified

Firefox 52

• Regressed Predictive prefetch makes duplicate HTTP requests in iframes, causing false ad impressions
• Regressed NTLM authentication may fail against HTTPS site
• Breaking Increased HPACK maximum table size breaks specific servers

Firefox 51

• Accept header for XHR has been simplified

Firefox 50

• SPDY/3.1 has been disabled

Firefox 49

Firefox 48

Firefox 47

• Accept header for images has been simplified

Firefox 46

• Regressed POST request fails on certain sites, showing connection reset page

Firefox 45 ESR

• Reverted jar protocol support has been disabled by default
• Line breaks in HTTP/2 headers are no longer allowed
• Undefined pseudo-header fields are no longer accepted in HTTP/2
• HTTP/2 draft is no longer supported
• X-Content-Duration header for Ogg media is no longer supported
• Breaking Firefox for Android UA string now has the Android version

Firefox 45

• Reverted jar protocol support has been disabled by default

Firefox 44

• Regressed Cross-site XMLHttpRequest is blocked if Service Worker is used without fetch event handler
• Reverted Whitespaces are no longer allowed in cookie names
• Line breaks in HTTP/2 headers are no longer allowed

Firefox 43

Firefox 42

• Regressed IDN URLs are not redirected properly
• Undefined pseudo-header fields are no longer accepted in HTTP/2

Firefox 41

• HTTP/2 draft is no longer supported
• X-Content-Duration header for Ogg media is no longer supported
• Breaking Firefox for Android UA string now has the Android version

Firefox 40

• Reverted HTTP auth dialog can no longer be triggered by cross-origin resources

Firefox 39

• Regressed 303 and 307 redirects after 302 are not followed

Firefox 38 ESR

• Language names in Accept-Language header are now always canonicalized
• Regressed Link with line break in hash part is broken
• SPDY/3 support has been removed
• Breaking HTTP/2 is now enabled by default
• Regressed Cache backend has been overhauled

Firefox 38

• Regressed Link with line break in hash part is broken

Firefox 37

Firefox 36

• Language names in Accept-Language header are now always canonicalized
• Regressed IDN URLs are not redirected properly
• SPDY/3 support has been removed

Firefox 35

Firefox 34

• Breaking HTTP/2 is now enabled by default

Firefox 33

• Regressed Files may be downloaded partially from misconfigured servers

Firefox 32

• Regressed Cache backend has been overhauled

Firefox 31 ESR

• Build ID in UA string is now frozen at 20100101
• Breaking HTTP connections now have a response timeout
• SPDY/2 support has been dropped

Firefox 31

Firefox 30

Firefox 29

• Breaking HTTP connections now have a response timeout

Firefox 28

• SPDY/2 support has been dropped

Firefox 27

Firefox 26

Firefox 25

• Build ID in UA string is now frozen at 20100101

Firefox 24 ESR

• Breaking Proxy-Connection header has gone
• Quality factor in request headers can now have 2 decimal places

Firefox 24

Firefox 23

Firefox 22

• Postponed RFC 2047 encoding support for HTTP header field parameters has been removed
• Postponed Third-party cookies are blocked by default

Firefox 21

Firefox 20

Firefox 19

Firefox 18

• Breaking Proxy-Connection header has gone
• Quality factor in request headers can now have 2 decimal places

Firefox 17

• A Preference to disable HTTP Keep-Alive has been removed
• Reverted Gecko build date in the UA string has been replaced with the version number
• Site-specific UA string now can be set

Firefox 16

• UA string no longer contains patch level version number
• WebSockets disconnection now follows the latest spec