I don't know, but I haven't seen the search change to Yahoo on my computer, except when I briefly tried Linux Mint many years ago. Mint had set Firefox and Chromium to Yahoo and made it extremely difficult to remove. It was both browsers, so I'm assuming that it was Mint that did it and not Mozilla.
Would you mind pasting your `about:support` somewhere (github gist, pastebin, etc) and linking it here?
Me? I'm not having trouble with Firefox. The Yahoo problem with Linux Mint was many years ago. My search engines in Firefox haven't switched on me since I uninstalled Mint.