Hi Mark

Firefox Quantum (The name given to the latest version of Firefox) has now been out for ages (In software terms) so no reason not to upgrade, there are 1000s of add on and extensions available for it and pretty much every previous one has been updated for it, (Or at least an equivalent is available) so the chances of the one you want not being available is virtually zero.
All modern video is embedded in HTML 5 (HTML is the code used to make web pages) as Flash has too many vulnerabilities in it, so while most browsers will still run it (Although you may have to activate it in the browser settings) it should be avoided if at all possible.
The latest browsers fully support HTML5 so there should not be any problems with YouTube, however there will be some sites that will not run as most pages these day use https rather than the old http as well as requiring certain security protocols that Windows XP does not have, (This applies to all browsers) so just be aware of this.

MS Works Word processor files (wps) can be opened in MS Word, spreadsheet files can be opened in Excel by saving them in xls format in MS Works first, the MS Works database files can be opened in Excel by saving them in CSV format in MS Work first. (If you still have the MS Works program it will run on Windows 10 by installing it using Windows compatibility mode (Right click the install file choose properties and follow the instructions in the compatibility tab) specifying Win XP)

Have fun and get off XP as soon as possible.

NOTE: Once you have transferred everything from your old computer and it is working properly on your new computer, you can use your old computer to try out a Linux distribution (Linux Mint is an ideal starter) and have some fun.

Bill