Today, for the update, I had a stale version of testcaseslist.json in my cache, so I had to empty my cache to run the new version. Correct uses of Expire and ETag headers should avoid this issue.
For this particular file, cache may not be necessary. It's very small (~3k to date) and will change at each version. The bottleneck isn't really here.
Looks like this involves Apache configuration and will need to be handled by Mozilla.
Think I've figured out a way to fix this without touching Apache's configuration:
Tested on a locally-hosted version of test262 which seems to work. I'll close the bug once I verify it works on the real website as well.
Seems to be fixed now against IE9 and IE10 PP2. Could you please confirm your browser is OK as well (there was a test262 update to the live website today).
I have received the most recent and haven't clear my cache or anything, so I think it works on FF5.