Firefox won't start after upgrading to Snow Leopard

I bought a new MacBook Pro recently, with Leopard installed. I went ahead and bought it before the Snow Leopard release, since Apple have a programme called 'Up-to-date' where you can upgrade to the new OS for a reduced price, £7.50 in my case. I figured I'd pay the £7.50 for six weeks' extra usage out of the laptop.

The laptop duly arrived, I migrated my account from my old MacBook over using Migration Assistant, and all was (mostly, but that's a topic for another post) well. Then the Snow Leopard DVD arrived. I upgraded, and all seemed good. The only problem I found was that Firefox wouldn't start. No errors, nothing. Just a couple of bounces in the Dock and then it disappeared. Starting up Firefox from another account worked fine, so I resigned myself to rebuilding my user account at some point.

Then, I tried Spotify. Spotify also refused to start, with a helpful 'Internal Error 1' message. Googling indicated that that this could be caused by Spotify being unable to write to its cache directory. This seemed strange, since I'd just installed it in a local ~/Applications directory - why would it be trying to write to a system location that it wouldn't have permission to touch?

However, the reference to caches provided a clue. A quick ls -l of ~/Library revealed that (goodness knows how) the Caches directory within Library had somehow obtained root ownership, so my own user account didn't have permission to write to it. This was solved with the following command, run as an administrative user:

chown -R dan:staff /Users/dan/Library/Caches

I have no idea how the Caches directory came to have that ownership. After correcting the above, Spotify runs, Firefox starts, and the whole OS experience seems quicker. Not hugely surprising, now disk caches can actually be written!