First, yes, Microsoft sites open fine in Firefox, but because of the do not allow extensions I have, I can't sign-in to any Microsoft sites from Firefox, which in turn drastically limits what I can do. And I'm very reluctant to remove my ad-blocking software from Firefox.
Secondly, when I right-click on a file, say with an .htm extension, I get a whole list of options, one of which is Open with..., which in turn opens a dialog box where both Firefox and IE are listed.
For other file types, its more complicated. Take for example .ini or .txt files: right-click, then Open with...then what opens is a dialog box offering Notepad, or choose default programs, which in turn opens another dialog box which offers Notepad and "Other programs", which if I choose "browse", opens Windows Explorer, which I then have to navigate to the needed program (at least 2 levels down), then select the executable file, then back out to the Open with dialog again, then.... Its a quite complicated, time consuming process.
And when I right-click on a link, I'm only offered my default browser (Firefox) or Copy link. So I Copy the link location, then open IE, then delete the home page address, then paste the copied link, then press enter....And then the page finally opens in IE.