This has been a favorite of mine for a very long time. It's the first thing I install on a clean machine (after Firefox and Greasemonkey, of course) and I've found it very, VERY handy.

I've never had any issues with it, ever.

Using Firefox 3.0.6, Greasemonkey 0.8.20090123.1, Textarea Backup seems to have no effect. Nothing is saved, nothing is reloaded, no additions to about:config.

This is a must-have script.

Saved lots of my nerves when dealing with slow or bad websites when they lose the information I entered into the form for last half an hour.

This script still sometimes triggers false alerts about restoring backups, but this is something I can live with.

Another cool improvement would be restoring not only textarea but edit fields as well, since some forms contain a mix of those types of fields.

I find the entries in about:config very disturbing...

But IT IS indeed a very good script, as Henrik N wrote.