For some of my browsers I do spoof the user agent, for others it is stock. Mull has Privacy Resist Fingerprint enabled for instance, this happens in all the browsers I have loaded. Mull, Smart Cookie Web Preview, Fennec, Iceraven, Cromite, Mulch, Fulguris, Monocles, Privacy, FOSS, DDG.
Just try a default Firefox. Though if you want to use TOR anyways, why not just use TOR browser? It's the only browser where the starting conditions are reasonably anonymous.