You can go to the profile folder and edit prefs.js, but it might be easier to create a plain text file named user.js and place this line in the file:

• user_pref("layout.css.devPixelsPerPx", -1);

Copy this file to the profile folder. See "Finding your profile without opening Firefox":

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Unfortunately there are, for some reason, 3 Firefox sessions open and I can't close any of them to restart Firefox

Unfortunately there are, for some reason, 3 Firefox sessions open and I can't close any of them to restart Firefox

The about:config page is a web page, so you should be able to zoom out the page via "Ctrl + -". How large is the text? Does it allow you to open about:config and paste the pref name in the search bar at the top of about:config?

The type is so big I can't even zoom out. I can't get to enter about:config. I also don't understand why 3 separate window are open with Firefox and I can't close them. My next step is to shut down and then restart my PC to see if that clears things. Please stay in touch.

If you reboot the computer and then start Firefox then you still have this pref value saved in the profile folder, so you will have to edit prefs.js or create the user.js file before starting Firefox.

Did you try the "Ctrl + Shift + Q" keyboard shortcut to close Firefox?

• https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly

I created the user.js file. Then shut down and rebooted. Then refreshed Firefox via Control Panel. Then opened Firefox and all seems to be well. Thanks for your help and patience!

You're welcome. You can mark the reply that worked for you as the solution, so others with the same problem can easier find it.

You can rename or remove the user.js to disable it. The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

• https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save