So far I'm up to 9.7GB uploaded and I'm having zero problems with Firefox 22.0 still. It is taking maybe 4-5 seconds to access some folders though, but with the amount of files on it already and that they're all images, that's to be expected.
Right now I have 27 tabs open, but I've had at least 50 open and still no problems. I thought maybe it was because I already was in the browse option, so instead of moving off that page within the tab, I opened up yet another tab for Copy. When I tried to access deeper folders, now that did crash
... and that's not surprising at all. But having just one tab on Copy still is working fine.
I have noticed though that a few of the subdirectories don't seem to be uploading or scheduled for uploading. Two small directories don't have any of the subdirectories shown for syncing (although their main directory is uploaded); another 2 directories haven't uploaded one subdirectory each, yet the other subdirectories have. I'm going by the change in the folder icons that Copy adds in Windows Explorer, and that the directory structures seem to have been uploaded first, except for these missing oddballs. Maybe they'll straighten themselves out with time as it gets nearer to completing the total upload, but I'll watch those. Or it might take a 2nd sync, it shouldn't but I'll keep watching.
The total upload is 28.3GB of all images between 1-8MB in size and a few much larger video files; 5,870 files in 29 subdirectories with another 124 subdirectories within those.
The app on my phone is still working perfectly as well.