Fully up-to-date CentOS 5.2 and Firefox browser. A previous thread had a problem with plugins disappearing. I've discovered, by accident, a causal relationship.
Running a Thunderbird mail client (could happen with others? Haven't tested with Evolution yet.) I click a link that opens the browser. BANG! The plugins.dat is "emptied".
Restored from my rsync backup, repeat, BANG! "Empty" again.
Now, if the browser is already open, NP.
Anyone can confirm?
Known problem?
Are you using a 64-bit firefox by chance? I have a similar problem with flash plugin. Try to reload a page or restart the browser. This works for me. I didn't look any further as I thought this to be a problem of 32-bit flash plugin with 64-bit browser.
HTH