I would start with using some internal drives for audio files.
If you don't use SuperDuper or something to clone your system, add that to your routine.
http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html
Your boot drive is getting punished with those hard restarts and the directory and filesystem plus files are probably getting scrambled. I'd order MicroMat TechTool Pro 4.6.x and/or Disk Warrior to straighten things out.
Something is corrupt but hard to know, or find, what it is other than to have an earlier backup from before the problems. And search Adobe for ideas. You should not have trouble with Firefox other than corrupt prefs, cache, or something. Cleaning out browser cache (or disable disk cache).
I keep the OS and apps on their own drive. My home account is on drive #2 along with media files. Scratch and editing on separate drive or partition. The whole thing runs smoothly.
Sometimes I've spent days trying to find out why, only to give up and just restore or reinstall the OS which was quicker and solved problem. Sometimes creating a new test user account is suggested to try to isolate it to personal home library or something.
I have to say that having an emergency boot drive is an essential that you can use to repair your boot drive and other drives, along with backups of your data, and of your "working system" (when it is back to working reliably).
Best bet to get through this is to focus on a fresh install on another drive and carefully update it to 10.5.4 and then backup; then install all your programs and make a second backup. Then gradually import and add prefs and files.
And watch the temperature of your RAM, drives, etc to make sure heat or high temps aren't a factor.