Download v54 from here;

https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86)

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Do Not remove the profiles folder.

Look for, and rename any Mozilla or Firefox program folders by adding .old to them. Do not rename the Mozilla Thunderbird folder if there is one.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. If all goes well, remove the OLD folders when you are done. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

Well there are many ways for you to uninstall Firefox, one way to do it is to go to your start button on the left corner of the screen and go to control panel and look for add or remove program. Or take a look at this article which tells you other ways as well.

Does Firefox create problems with Avast, or does Firefox crash because of Avast? I'm not sure what the incompatibility is that you're experiencing. There are many users who have both Firefox and Avast. Problem occur from time to time (you'll see that if you search for Avast here), but usually everything can be made to work together.

I installed Mozilla 55.03 but I'm having problems uninstalling Mozilla 54.01

Hi marymora, do Firefox 54 and 55 have different entries in the Windows Control Panel, Uninstall a Program? Is that the method that isn't working?

There is an alternate method listed in the article that another reply linked to: Uninstall Firefox from your computer.