User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:65.0) Gecko/20100101 Firefox/65.0
Steps to reproduce:
Uploading large files (500MB+ but seems to get worse as they grow larger) to for example http://www.sprend.com/ but I've had the same issue on many other websites.
The upload often hangs at 100% and eventually times out. Retrying up to 5 times usually gets it to work. This happens since Firefox Quantum, I still have older versions of Firefox on the side and they never cause this problem, still.
The upload finishes and I get redirected to the upload complete page.