Not at all. Personally I like to have a shared data partition for my media files. Both OSes can use that just fine, if it’s NTFS or exFAT formatted.

OnlyOffice hides the fact that it’s Russian.

And is it fully compatible? Most Excel-alikes have at least some hiccups.

Firefox has effectively become spyware, and so, yes, it should be replaced. There are several forks that work well. I’m using Floorp (stupid name, but really nice with some improvements over vanilla Firefox). Zen is another one gaining in popularity. Try them out! Don’t let Mozilla walk away with your data.