As far as I can tell (with addons.mozilla.org not great for finding things):

Search all Tabs is the best of three comparable extensions.

https://github.com/Drive4ik/simple-tab-groups/issues/759#issuecomment-787096598

No display of the results of the search. Only the title of the pages. What is the use of this plugin if you cannot see the result of your search across multiple tabs.

Quite useful. However, it is somewhat slow to load. Please fix that.

Better than the others I tested.
Instant list showing context for open tabs, +title only search for unloaded tabs. Jumps to highlighted text from list.
Feature request - WB great to search body of unloaded tabs - in cache???

Really one of the tool in the swissknife. Must have add-on for people with multiple tabs and looking for a needle in haystack.

Yet there's room for improvement

Thanks for updating the addon. Many improvements and intuitive popup interface.

Compatible with Waterfox! This add-on compiles data of all loaded tabs, meaning two things: (1) unloaded tabs won't be searched and (2) it takes a while for the search engine to be ready after restarting the browser. Actually, some times it simply doesn't work, at all. Disabling and re-enabling it helps, some times, but not aways. Also, the search pop-up must be closed before searching for newly open tabs (the add-on will conveniently inform how many tabs will be searched on the pop-up). It has no options whatsoever, what is not really a problem, as it works perfectly out-of-the-box, but could have a few tweaks, like an option to search only on the current window's tabs, whole word/phrase search, regex search, case sensitive search and whatnot. Anyway, it's still an amazing and fundamental tool for anyone with more than a half dozen tabs open.

At last a tool that searches ebay tabs.