[CentOS] [OT] Firefox loses plugins, anyone else? Bug? Known?

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?

If not, I'll post bugs in the appropriate places.

TIA.

-- 
Bill