@MacBH928 For clarity's sake, I would like to emphasize that foxPEP and TenFourFoxPEP are two different patch-scripts intended for two different codebase pools, with the latter being intended solely for the TenFourFox variant of the Firefox 45 codebase, and the former being intended for the vanilla Mozilla variant of the Firefox 52+ codebases, including Firefox Quantum. With that being said -
1. Yes, it works just as well with modern hardware as vintage hardware. And as previously stated, it should remain compatible with Firefox and its derivatives for the rest of their development lifespan. It can be tested by simply installing it into the browser's user profile folder normally.
2. Assuming that you are referring to every subsequent update of Firefox, reinstalling foxPEP every several update iterations should be sufficient to minimize internal preference clutter as the versions stack up ... although even that may be unnecessary.
3. foxPEP should technically be compatible with the Tor browser, however I would suggest approaching with caution, as it was never tested nor intended for use with Tor, and I would in fact (due to said lack of testing and therefore knowledge), actively recommend against pairing it with Tor for anything besides casual, recreational, or experimental use in the event that the stakes are critically high for maximum security and user privacy.
Make no mistake though that foxPEP still objectively improves on the security and privacy that Mozilla offers in vanilla Firefox by a large margin. This recommendation is simply a precautionary measure teetering on paranoia, and nothing more.
Thank you for your interest.