To access the sqlite databases, you need to do a little searching: on my system these files are on
/Users/markw/Library/Caches/Firefox/Profiles/btzzru2q.default/Google Gears for FirefoxNotice the randomly generated directory name. I plan on searching for "Google Gears for Firefox" and using its path. I was able to use sqlite (version 3 required) to open the two top level ".db" files and the attachments and emails themselves are stores in nested directories; for example, on my system, I am looking at
ls -asl ~/Library/Caches/Firefox/Profiles/btzzru2q.default/Google Gears for Firefox/mail.google.com/https_443/GoogleMail\[12\]#localserver/10and I see the HTML text for several emails and their attachments.
Before I stop using my old backup scheme I want to feel secure that I can access old emails independently of GMail. I may keep using my GData Google Docs script because I can convert all of my documents to OpenOffice.org format before downloading and archiving them.
In any case, local persistence with Gears should help Google convince more people to rely on their "office in a cloud".