The "Tab Previews" are supposed to show below the tab. Wherever they are they're a nuisance and can be got rid of.

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 into it. That will then appear below with the word true next to it. Double-click true and it will change to false.

The Pillenwerfer:

Bless you! Thank you so much! I have used Firefox forever. This issue almost made me switch to Safari.

Again, THANK YOU  !!!!!!!!!!!!!!!!!!!!!!!

There are two Boolean (true/false) prefs in about:config to control what to show when you hover a tab. This pref controls whether to enable this new hover feature or only show the legacy tooltip with the title.

• browser.tabs.hoverPreview.enabled

This pref controls whether to show a thumbnail or a two line tooltip with title and URL.

• browser.tabs.hoverPreview.showThumbnails

Showing the thumbnail can be controlled via "Settings -> General -> Tabs -> Show an image preview when you hover on a tab".

Note that such a main enable/disable control pref is only meant for developing a new feature and it will be removed once this feature has been finished, it isn't guaranteed how long it will last.

• 1914155 - Remove the browser.tabs.hoverPreview.enabled pref

(https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

The showThumbnails pref will stay since there is a setting in the UI that controls it.

I understand it may be removed, but like the OP, I too would leave Firefox for Safari over this issue. It really makes the browser unusable. I don't know what is happening with UI development at Mozilla, but they seem to have lost their way. In addition to this issue where the tabs are blocked by essentially redundant information, and then on top of this Mozilla is also trying to display a thumbnail of the page which also covers up the tab name. Just dumb.

The other key UI issue I continue to wrestle with (not looking for a solution), is how Mozilla now pastes the large "you need to update" info box on top of your screen and it can not be dismissed or minimized. It visually blocks you from continuing to use the browser until you shut down the visible tabs (you can't access the tab under the large info box many times), and update the browser. I could understand this if their updates were infrequent, but they aren't. It has gotten to where I spend about half my time fighting or performing "less than useful" updates, rather than doing the work I want to do. That greatly diminishes the value of a browser if I have to spend so much time maintaining it.

Firefox has slipped to 3rd or 4th place in the browser wars as a large number of users have left for Chrome, Safari, or Edge in large part I believe, due to dumb UI changes. Firefox use to be highly configurable, which is what many users loved about it. That is no longer the case. Just saying.

I've already found how to remove them with custom CSS for if/when the preference goes, though hopefully that won't be needed.

You still can hide the thumbnail and will be able to keep doing this via "Settings -> General -> Tabs", so you do not need to access about:config. This will leave you with the two-line title and URL tooltip, where you previously had a tooltip with a dark background.