Ola,
Iedereen die ook FireFox gebruikt als browser en zijn mailtjes lekker leest in Outlook kent het probleem, alle linkjes die je aanklikt in de mail openen in IE! Kut! Daarvoor het je FireFox niet…
Hieronder de oplossing! 
1. Launch My Computer or Windows Explorer.
2. Choose menu `Tools` -> `Folder Options`
3. Click the `File Types` tab.
4. Scroll down to find `URL:HyperText Transfer Protocol` (under the File Types column) and make sure it`s highlighted.
5. Click the `Advanced` button.
6. In the `Actions` box make sure that `open` is highlighted (click on it if necessary)
7. Click the `Edit` button.
8. Uncheck `Use DDE`, click the Browse button and browse to Firefox.exe on your system, and click OK. [Updated for clarification, sorry about the earlier omission.] If you get a “file not found” message, browse to Firefox.exe and click OK.
9. The path to Firefox.exe should now appear in the `Application used to perform this action` box.
10. Go to that `Application used to perform this action` box, and add `-url “%1″` at the end so that it reads e.g. (on my computer at least) “C:Program FilesMozilla Firefoxfirefox.exe” -url “%1″
10. Click OK again.
11. Repeat the process with the file type `URL:HyperText Transfer Protocol with Privacy`
Yippy! 
bron: hier
