From MozillaZine Knowledge Base
This article describes how to go back to a old version of Firefox. See this article if you want to go back to an old version of Thunderbird.
To downgrade to an old version of Firefox, back up your Firefox profile first, as a precaution, and then follow these steps:
- Download the version you want from http://download.cdn.mozilla.net/pub/firefox/releases/ or http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/ Note: If you are looking for an older version of a Firefox "final" release, look for a download with no letter suffix (e.g., 18.0.2 is the last final release version previous to 19.0). Release versions that end in esr (e.g., Firefox 17. 0.3esr) are Extended Support Release versions intended for organizations. Versions that end in b (e.g., 19.0b6) are Beta releases, intended for users who are testing pre-release versions.
- Exit Firefox and then uninstall it. Make sure to remove the Firefox installation directory.
- Install the older Firefox version. You can install Firefox in the same location to minimize any side effects. For example, the Windows registry or other installed programs may expect Firefox to be in the default location (e.g., C:\Program Files\Mozilla Firefox or C:\Program Files (x86)\Mozilla Firefox on 64-bit Windows).
- After installing the older version, turn off the automatic Firefox Software Update feature (if enabled).
- Reconfigure your firewall if you have one.
- If Firefox disables any of your add-ons you probably need to replace it with a version that supports a older version of Firefox.
- If you experience other problems, create a new Firefox profile and transfer your data to the new profile.
Previous released versions of Firefox are no longer maintained and become obsolete. Although you can still download these old, obsolete Firefox versions, they contain security vulnerabilities and are unsafe browsers.
There are real security problems (actual exploits in the real world) with old versions of Firefox. See the Mozilla Security Advisories page for more information about vulnerabilities in old versions of Firefox. Rather than downgrading to an older, unmaintained version, you should consider switching to the latest version of SeaMonkey, Google Chrome or Opera. You should also consider installing the latest Extended Support Release version.
If you must install an old version of Firefox: