reblogging with my (edited/expanded) replies cause this actually got my dash loading properly again.
for desktop Firefox, go to Settings -> "Privacy & Security" -> "Cookies & Site Data" -> [Manage Data…], select Tumblr.com from the list, click [Remove Selected] and [Save Changes] (this will log you out of Tumblr)
for mobile Firefox, go to Settings -> "Delete browsing data" -> ensure that ONLY "Cached images and files" is checked, then tap [Delete browsing data] at the bottom. (this will affect ALL sites, so you might want to be on wifi if you're worried about network/data use)
Given that my cookie-managing extension didn't see the 1.5 gigs of data I had to delete on my desktop, the site is likely caching a bunch of dumb shit for way too long for some reason and not like, building up a massive cookie.
I know, I know - I'm being pedantic, but I think it's worth yelling about the right thing if you're gonna start yelling (and we probably should be yelling...)
Anyway, Tumblr's been caching something while we scroll our dashboards. Could it be like, every image? Just thumbnails? Maybe, but I think that would balloon WAY faster. I hate gambling, but I'd bet that they started caching post IDs/timestamps/positions/etc for that silly "Now where were we?" pop-up that I've always ignored because I was refreshing my dash to get new posts. And eventually we end up with over a gigabyte of pointers to posts and flags as to they were rendered on our screen or not, bogging down your machine every time to refresh the page. But someone more adept at digging into things can confirm or deny that.