Only the Firefox releases that supported Windows XP work on ReactOS, which was dropped in Firefox 53 in 2017.

Modern versions don't. There was an LTS version for XP that was updated up until relatively recently that still works, but I wouldn't recommend using that for a daily driver anymore.

In theory someone could take modern Firefox, figure out what essential APIs are missing, and contribute a set of fixes to get the latest LTS version of Firefox working again, but that's quite a complicated process.