Firefox used to have good hardware acceleration in XP... A year or two after it was introduced in version 28 or so, they disabled it. Chrome did the same thing, but there are hacks to re-enable it on later versions.
It has to do with some of the DLLs removing support for DirectX9.
I believe this hack can be ported to Firefox, though I asked the author about it it (svyatpro) and he was only successful with Chrome.
A good proof of concept is the HWACCEL test: HWACCEL.zip (maybe someone can upload it to the forum for posterity, it's over 2MB).
The reason I bring this up is because of the planned obsolescence talk, with developers intentionally removing support for older operating systems... Firefox did this too back in the day. Soon these developers will force people to use Windows 8.1 or Windows 10... ***shudder***