Type about:config into the search/URL bar.

That will bring up a warning which you'll have to acknowledge.

On the next screen there's a search box at the top. Type or paste browser.tabs.hoverPreview.enabled. That will then appear below with the word true next to it. Double-click true and it will change to false.

Note that tab preview is accessible via "Settings -> General -> Tabs -> Show an image preview when you hover on a tab".

Do you lose other settings or only this preview thumbnails?

Do you have numbered prefs-##.js files in the Firefox profile folder?

You can use the button in "Help -> More Troubleshooting Information" (about:support) to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Setting up a user.js file in your profile may help to make the change stick.

Open Notepad, or whatever, and paste this into it:—

user_pref("browser.tabs.hoverPreview.enabled", false);

Save it as user.js and check it really has and not user.js.txt — right-click > Properties is one way — and then drag it into your Profile directory.

I've never had to re-set it since the horrible thing was inflicted upon us.