I'd guess those 2300 messages you told Thunderbird to keep already fill up your (small) storage space. May be you should look for a new email provider with a decent online storage space.

I have deleted a lot of emails in Thunderbird, checked my gmails online and they match. But it's still saying I've used 82% of my storage (15GB) - is that my total google drive storage it's looking at? B/c that's true that my drive for that main email account is about 82% full with pictures and documents. But I have a 2nd gmail in my thunderbird window as well and it's nowhere near full. So where is the 15 GB coming from?

drmeow,

please start a new topic for your problem. https://support.mozilla.org/questions/new