I finally had time to go on with my project of deleting old attachments from my Gmail account and seemed to be making headway, until I noticed that something was wrong.
As I went through my old messages in Thunderbird, deleting attachments like crazy, I would keep Gmail open in my browser window also. I'm not sure how the folders in Thunderbird sync with Gmail, so I'd take breaks between batches of deletions and use Gmail's web interface to remove the newly stripped messages from my "Check Attachments" folder.
While I was doing this, I would occasionally look at the counter in the lower-left corner of the web UI. That's where it says "n% full" and "Using x MB of your y MB."
I expected these numbers to drop as I went on, but they were actually increasing. At first, I thought that it was just because I was receiving a lot of new e-mails and attachments, and I wasn't deleting old ones fast enough. Today, though, I saw the number go up (1) while I was deleting and (2) while there were no new messages for hours.
I did a quick search of a few stripped messages, and this is what I found:
For some reason, Thunderbird deletes the attachments, and then it (or Gmail; not sure which) creates a complete duplicate of the original messages with the attachments intact. So instead of reducing, the bug actually almost doubles the amount of space used by these old e-mails and files. :/
I've since filed a bug report and put this project on hold, while I go through all these e-mails and delete the duplicates one by one.
The dupes only show up in Gmail, by the way; they're not visible in Thunderbird, so add-on applications that claim to remove duplicates don't detect anything.
I've created this blog post in case anybody used my prior Gmail notes (sorry, Zoe) and they have the same problem. I'm also hoping to get the attention of someone who knows how to fix this, since within several seconds, my bug report got buried under all the new ones. :(
UPDATE: Someone responded to the bug report and said that it was likely a Gmail problem, not a Thunderbird problem. In the meantime, I've taken to this only-slightly-more-tedious way to delete old attachments:
1. Delete attachments in Thunderbird.
2. Search Gmail using the web UI for the just-stripped message.
3. Delete duplicates via the Web UI.
4. Remove stripped messages from Gmail label "CheckAttached."
3. Rinse and repeat. For ~4,500 messages with attachments.