Sure that's great, but that should either be separate from the address bar or at the very least shouldn't be so 'default' for lack of a better term, that just clicking without thinking activates that behaviour. For people like yourself that use it to its full advantage I won't argue that is a good feature. For the majority of people trying to make a simple search, it doesn't seem so helpful.
I feel like default behaviour should be broadly helpful, while specifically helpful behaviour should be easily accessible but not so easily accessible that a mindless click triggers it.
On my Firefox 79, if I pop open a new tab, 1) the cursor is refocused from the search box to the URL bar and 2) the URL bar is a search box (even if search-in-url-box is successfully disabled in not-new tabs).
Even better, the option to set new tab to a webpage is gone. The only choices are the Firefox default page or a blank webpage.
Normally this wouldn't be a problem. I'd just try one of the dozen of known tweaks to about:config or userChrome.css and defeat this behavior. Except none of them impact this.
FWIW, and as a workaround and not any sort of defense of FF breaking itself: You could, from an existing page, type what you want in the search box, then middle-click (or ctrl+click) the search button, which opens the result in a new tab. Again, you shouldn't have to do this, but it seems like it'd patch your workflow.
In the end I opted for an extension - it redirects a new tab to a web page of my choosing.
The action is a bit distracting but it happens fast enough that it's not a bother.
I'll note it was the 2nd extension I tried. It may be some onerous Firefox security measure needs to be bypassed & the first extension hadn't managed it yet.
I feel like default behaviour should be broadly helpful, while specifically helpful behaviour should be easily accessible but not so easily accessible that a mindless click triggers it.