You can have them both on your computer and use which ever you want. I've been running them both on XP for years.
To export favorites
Follow these steps on the computer that you're exporting your favorites from.
Open Internet Explorer.
Click the Favorites button, click the arrow next to the Add to Favorites button, and then click Import and Export.
In the Import/Export Settings dialog box, click Export to a file, and then click Next.
Select the Favorites check box, and then click Next.
Select the folder that you want to export your favorites from, and then click Next.
By default, Internet Explorer creates a file called bookmark.htm in your My Documents folder. If you want to use a name other than bookmark.htm or to store the exported favorites in a folder other than My Documents, specify the new file and folder name.
Click Export. If you already have a file by the same name, Internet Explorer will ask you to replace it. Click Yes if you want replace the file or click No and type a new file name.
Click Finish.
then import them in Firefox.
addresses are saved in an address book - depends on which e-mail you use.
groups are stored on yahoo - that doesn't change anything
I have been using both for years as well. I just have shortcuts on the screen for which browser I want to use. Explorer 9 has really caused me a lot of grief with HTML programs, etc. So I use FireFox for those.