Hi Rob
You will find the answers to all your questions here:
www.applematters.com/article/switch-to-a...youll-never-go-back/
I will summarise below with reference to your specific queries.
Your Windows version of Word will not run directly with a Mac but it can be run be run using Boot Camp that comes with the Mac. This will require a Windows Install disk. You can use this to run any Windows software that doesn't work on the Mac. You can use VMware or Parallels software to run Windows programs from the Mac OS.
The alternative is to buy the Mac version of Word or download the Free Mac version of Open Office which is fully compatible with Microsoft Office.
There is a Mac version of Firefox (my favourite too!).
All your files will work fine on a Mac (subject to compatible software).
The main downside is program compatibility but this is now manageable (in the past it was more difficult). Unless you are particularly attached to a piece of Windows software you will find all you need in Mac World. There are less programs written in Mac World but I suggest there are enough!
The main advantages are beauty, performance, security, stability, functionality, ease of use.
This brings me to security. This is covered in the link above but the Mac OS has been more secure since inception (it had the advantage of being able to learn from Windows). Windows is handicapped by its poor initial design and having to patch it rather than redesign because of legacy issues. Having said all this Windows 7 is very promising and addresses many of the issues associated with Windows.
I think Windows 7 is aptly named because I give it "7/10 Good effort". Mac gets a "9.5/10 Excellent but could make more effort to play nicely with others"!
For the record: Vista: "4/10 You've got to be joking"; XP "6/10 Could try harder".
I hope this helps
Steve