Reset the page zoom on pages that cause problems.

• View > Zoom > Reset (Ctrl+0 (zero); Cmd+0 on Mac)

• http://kb.mozillazine.org/Zoom_text_of_web_pages

You can rename the file content-prefs.sqlite to content-prefs.sqlite.old (or delete the file) in the Profile Folder to reset the page zoom on all sites.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

I've tried that, but it still doesn't work.

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

If you have increased the minimum font size then try the default setting "none" as a (too) high value can cause issues.

• Tools > Options > Content : Fonts & Colors > Advanced > Minimum Font Size (none)

Make sure that you allow websites to choose their fonts.

• Tools > Options > Content : Fonts & Colors > Advanced > [X] "Allow pages to choose their own fonts, instead of my selections above"

• https://support.mozilla.org/kb/Changing+fonts+and+colors

It is better not to increase the minimum font size, but use an extension to set the default page zoom to prevent issues with overlapping text.