It may become necessary to downgrade Firefox to previous version if an automatic update prevented the browser from working. Older computers may not have the specified requirements to run a new version of Firefox, and an update can easily prevent the browser from working. In this case, it will become necessary to downgrade Firefox. Fortunately, Mozilla keeps the previous versions on an FTP site.
Something you should try before downgrading the web browser is disabling any add-ons. The Firefox update might not have broken the browser at all. It could be that one of the add-ons you’re running is not compatible with a change that was made in the update. Before downgrading Firefox, try disabling your add-ons first. To do this, go to Tools>then Add-ons and disable them. If Firefox begins working, try enabling the add-ons one at a time until you find the one that’s causing the problem.
If the browser still isn’t working, write down the version by going to Help>About Mozilla Firefox. Then proceed to uninstall it via the Control Panel.
Note: You might want to backup your Bookmarks before doing so. A quick way to do this is to Import them into another browser, such as IE or Chrome.
Click on the link of the release you want to download.
You will then see folders for all of the various Operating Systems this particular releases is available for. Choose your Operating System, download and install that version of Firefox. You will then have successfully performed a downgrade of Firefox.
Running the most current version of any browser is preferable. You should try and determine why the upgraded version failed to run on your system and take appropriate actions to correct the issue.