I assume this problem is on your Mac and not on an iOS device like an iPhone or iPad.

For question-mark-fonts on Firefox 56-57, see: Fonts on many websites show up as a question mark in a box.

Regarding your search engine default changing, one possible culprit would be an extension. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Command+Shift+a (Windows: Ctrl+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then cast a critical eye over the list on the right side. Any system extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious -- mentions search, privacy, or isn't something you remember installing? If in doubt, disable.

Any improvement?

Well, I followed all your instructions. Disabled all the add-ons, extensions and nothing changed. Went and change the value from 3 to 1 and some of the text appeared but the rest is literally in Greek! Now, what do I do to convert from Greek to English?

I am on a Mac with Sierra 10.12.6.

Does your Firefox request pages in Greek? You can check on the Preferences page:

• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Windows: "3-bar" menu button (or Tools menu) > Options
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

Scroll down to the Language and Appearance section, and click the Choose button. Does this dialog list Greek preferred over English?

If that setting is correct, Firefox has some kind of problem with your fonts. I have limited Mac experience, but it sounds as though you are using some kind of third party font manager. Can you turn that off temporarily or is it essential for your work?

Language preference is set to US English. Yes, I use FontAgent Pro and have used it for more than 15 years and always on this machine. This is a recent issue (with recent FireFox updates). I will try to disable font manager and see what happens. However, I do need it for my work. Can I go back to an older version of Firefox?

After disabling FontAgent Pro and trying a few times. The Support screen and websites rotated between Greek and what looked like Spanish or French, it finally went to English. Now, I have to figure out how to make it work using the font manager or drop FireFox and use Chrome or Safari. Such a shame.

What was the last version of Firefox that was working normally for you? Or to ask that another way, when did this problem start, and then you can compare against your update history on the Troubleshooting Information page. There's an update history button in the first table on the page. Either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return