Startup crash with RelevantKnowledge adware installed. Firefox Support has helpful instructions to remove it. (bug 1394550)

Startup crashes with 64-bit Firefox on Windows 7, for users of Lenovo's "OneKey Theater" software for IdeaPad laptops. To fix this crash, please re-install 32-bit Firefox. (bug 1369361)

Users running Firefox for Windows over a Remote Desktop Connection (RDP) may find that audio playback is disabled due to increased security restrictions. Learn how to mitigate this issue until it is corrected in an upcoming release.

Due to a bug in Mac OS X High Sierra, fullscreen mode has some issues (bug 1390125)

GSSAPI and Kerberos authentication do work in 64-Bit Windows Firefox. However it requires the advanced setting "network.negotiate-auth.gsslib" to contain the string "gssapi64.dll" because the default value (which is chosen if left empty) is wrong. Future versions of Firefox will contain a correct default value. (bug 1409275)

Some third party software (Comodo Internet Security, Kaspersky, Quick Heal Antivirus) are known to cause issues with Firefox 64-bit. These vendors have published new releases which addresses the issues. (bug 1407712)

Some older versions of the ATOK IME, including ATOK 2008 and 2009, can cause crashes or may not work on the Windows 64-bit version of Firefox. To fix those incompatibility issues, please use a newer version of ATOK or one of other IMEs. (bug 1409155)