Firefox Legacy is an unofficial modified version of Mozilla Firefox that works on older Mac OS X versions, including 10.6 Snow Leopard, 10.7 Lion, and 10.8 Mountain Lion.
Download it now and breathe new life into your old Mac.
As of 2021-10-24, Firefox Legacy is no longer supported. It has become increasingly challenging to update it to a more modern Firefox version, and I no longer use it myself.
There will be no further updates to Firefox Legacy.
For an up-to-date browser for older Macs, try Chromium Legacy.
To enable Google sync in Chromium Legacy, quit the browser, then enter the following command in Terminal (it is one long line):
open -a /Applications/Chromium.app --args "--oauth2-client-id=77185425430.apps.googleusercontent.com --oauth2-client-secret=OTJgUOQcT7lO7GsGZq2G4IlT"Chromium Legacy will open; log into your Google account. You will not need to run the command again unless you log out.
Final version for OS X 10.8 Mountain Lion: 71.0p2 (2020-09-15)
IMPORTANT NOTE ABOUT UPDATING TO v71: A new Firefox profile will be created when you update from earlier versions to version 71.0. To restore your previous data, type "about:profiles" in the address bar, then find your previous profile (if you can't tell which, try each one) and set it as default. Then, restart Firefox Legacy. I apologize for the inconvenience; the cause of this issue is not known.
Final version for OS X 10.7 Lion: 68.12.0p3 (2021-01-24)
Now featuring support for numerous newer web standards, including Push, Service Workers, Feature Policy, ResizeObserver, Promise Rejection Events, more of the Animations API, and several CSS features. This should improve compatibility with many websites.
For security and privacy reasons, starting in version 68.12.0p3, support for TLS 1.0 and 1.1, HPKP, and the Battery API is disabled.
Only Firefox 52.9.1 is available for OS X 10.6 Snow Leopard; it will receive no further updates, in part due to technical limitations of the OS.
Note: the program is called "Nightly" due to licensing restrictions prohibiting the use of the official Firefox branding.
Final version for OS X 10.6 Snow Leopard: 52.9.1p1 (2020-04-11)
Any of the language packs for the matching official Firefox version are supported.