For some reason, and I really have no idea what I could have possibly done to make this happen, any time I use Firefox to try to download anything at all, I get an error message:
I couldn’t even find an AppData folder in my user directory…I have no idea why this is showing up all of a sudden. Any help?
Something odd has probably happened to the file permissions for your TEMP directory. See if a reboot fixes it. It is also possible, but unlikely, that your C drive is full and there isn’t enough room to write the file to the temp directory.
AppData is a hidden directory. You’ll only see it if you have Windows Explorer (file explorer) set to “Show hidden files and folders”. You can also see it if you open a command prompt and type “cd %temp%” (without the quotes).
Rebooting didn’t help. and I still have over 125 gigs left on my C drive.
mozillaZine has an article about the error, but the article is about FF2 and not FF3: Source file could not be read
The mozillaZine forum also has a thread with no clear resolution: issues with firefox 3.0.1
One possibility is your virus scanning software messing things up by locking the file. Try disabling the on demand file scanning in your antivirus software. Then try a download.
Where are you trying to download your files into? I have mine sent to my Desktop… the .part files show up there, too. Never had a problem.
Check out Tools->Options->Main->Downloads
I had set it to always ask me where to download things to, that was one of my first attempts at solving this problem.
Still not working. I’ve actually opened IE a few times to download some things I needed. 
Perhaps you should write the Mozilla Foundation and ask for a refund? 
Just wanted to update, firefox prompted me to update to v3.0.2, and the problem is fixed, lol.
I have had some a few rare troubles with firefox
It’s good to see you are fixed now.
I was going to suggest uninstalling firefox. It will ask if you want to save bookmarks and settings. Of course click yes. In a minute it’s gone. Then reinstall it . Takes less then a minute. Both times I did this, firefox was fixed, and I still had all my bookmarks, the forum still knew who I was etc.
Sometimes it’s better to flush the toilet then to try to figure out whats crap and what isn’t.