I'm using the main line firefox release on MacOS here, and yeah it does seem to do that... Clearly google already knows what browser I'm on from useragent as the most rudimentary ID and other browser fingerprinting measures as well.

(edit: one theory would be they're tracking how many URLs are copy/pasted and shared by firefox users to non-firefox users and then loaded by people with different browsers, there has to be some value to them in that data in the aggregate of millions of search page loads).

I figured leaving it into the copy and pasted URL is harmless and ultimately mozilla derives the bulk of its revenue from its search deal with google, so it's also a way of supporting continued firefox development in the most minuscule way.