While I was doing some browsing the disk in my laptop started thrashing.

I left it for ages, but it just never stopped, and the whole machine became unusable while it was doing it.

After about 1/2 hour, I gave up, and I had to power it off by holding the power button, nasty.

When I booted back up, I couldn't start Firefox, it was complaining that the profile was in use.

I looked around, went into .mozilla/firefox in my home directory, and I had a look through the profiles.ini file. I worked out which profile it was using, and was the path was.

I went into the directory set as the path, and saw there was a lock file in there, so I trashed it, and then Firefox started without an issue.