I am not a fan of Win 10, and was a Preview tester since last October. That said, there is a lot of misinformation floating about - among that regards FireFox.
I had NO problem in running FireFox as my web browser. It was my default web browser when running Win 10. No problems were noted along the way.
Perhaps some of the reported difficulties involve how Win 10 was installed. The upgrade method is the "easy way out" and is popular among the less skilled computer users. It is not the preferred method, which is a clean install.
Sure, you have to do a little work, such as reinstalling your various software(s) - but you end up with a system carrying no baggage.
So far as your personal files, data, info, whatever - save it on a thumb drive before you start any changes. Then you can copy from that device to the computer running Win 10 - after you have it all set up and running.
If you do not understand what I have said - then I suggest you proceed with the upgrade method and take your chances. This is not meant as an insult - it is just a statement of fact.
Personally, if you are running Win 7 and are happy with it, I would delay installing Win 10 for at least 6 months - to allow for many of the bugs to get fixed. That is my plan at this time.
Of note, my primary everyday computer for Internet, email, spread sheets, etc. runs Linux Mint. I run HRD, TurboTax, etc. on my Windows machine - as none of the aforementioned have the ability to run on Linux.
Bill W2BLC