Hi Paul,
You should be able to Install Firefox without loosing anything, but I'd first get Applejack...
http://www.versiontracker.com/dyn/moreinfo/macosx/19596
After installing, reboot holding down CMD+s, (+s), then when the DOS like prompt shows, type in...
applejack AUTO
Then let it do all 5 of it's things.
At least it'll eliminate some questions if it doesn't fix it.
The 5 things it does are...
Correct any Disk problems.
Repair Permissions.
Clear out Cache Files.
Repair/check several plist files.
Dump the VM files for a fresh start.
Then... Try putting these numbers in Network>TCP/IP>DNS Servers, for the Airport Interface...
208.67.222.222
208.67.220.220
Then Apply
DNS Servers are a bit like Phone books where you look up a name and it gives you the phone number, in our case, you put in apple.com and it comes back with 17.149.160.49 behind the scenes. 🙂
These Servers have been patched to guard against DNS poisoning, and are faster/more reliable than most ISP's DNS Servers.