Not much more I can add to what Lockherup is telling you--that's on-the-money advice! I would advise that if you want to use two different versions of Firefox, you have a lot of XP-compatible versions to choose from. Here are some milestone versions, actually:
-1.5.0.13pre (last version designed for Win95 and [IIRC] NT 3.51)
-2.0.0.22pre (last version designed for Win98/98SE/ME and NT 4.0)
-3.6.29pre (last version to have a classic Firefox feel [no orange FF button])
-4.0.2pre (last version before Firefox switched to a rapid release schedule)
-10.0.12 ESR (only ESR designed for Windows 2000)
-12.0 (last version designed for Windows 2000)
-17.0.11 ESR (last ESR to have JavaScript toggling accessible from the options menu)
-22.0 (last FF version to have JavaScript toggling accessible from the options menu)
-24.8.1 ESR (last ESR without the Australis interface)
-28.0 (last FF version without the Australis interface)
-31.8.0 ESR (last ESR to have the old-school pop-up options menu by default)
-37.0.2 (last FF version to have the old-school pop-up options menu by default)
-38.8.0 ESR (first ESR AND FF version with tabbed preferences, though the pop-up options menu can be resurrected via about.config)
-45.9.1ESR (last ESR to not require a processor with SSE2 support)
-48.0.2 (last FF version to not require a processor with SSE2 support)
-51.0.1 (last FF version to support all NPAPI plugins [not just Flash])
-52.9.1ESR (last ESR AND FF version designed for Windows XP)
If I'm not mistaken, there are a few more milestones worth mentioning here, but this should pretty well cover the major milestones.
Of course, I am going strictly off official Mozilla releases, though some of these are nightlies and Tinderbox builds. I don't count stuff like roytam1's browsers (at least for now; once they're more stable and use less RAM then I might be more inclined to). If you want to have two FF versions on your PC, then my two choices would be 28.0 and 52.9.1ESR. 24.8.1ESR is newer than 28.0, but it is lacking in web compatibility by comparison (for example, Twitter [the full/main desktop site, not the legacy/mobile versions] only starts to work properly with 26.0, but that too has trouble with a decent number of modern sites; 28.0 is when Mozilla's rendering really started to catch up with today's web standards, though sadly it coincided with Mozilla ushering in Australis with 29.0). 28.0 should work fine for most sites, and 52.9.1 ESR can well pick up the slack on ones that it may be starting to struggle with right now.
Not sure what more I can say here...hopefully some might find this info helpful!