> What do I do if as a normal user
That really depends how "normal" the user is.
If you have basic HTML/JS/CSS skills, you can try to minimize the testcase - https://css-tricks.com/reduced-test-cases/
If you don't, you can always contact the website author and inform him about performance problems and ask him to get in touch with us. He does have access to the whole website and could help us get a temporary account for us to investigate.
If you have more knowledge and curiosity, you can even use our performance devtools to generate a profile. It's actually fairly easy - just start recording in the performance tab and do what you would normally do that is slow.
Then file a bug and attach the profile.
This will give us an insight into what takes so much time. It may not be enough, but sometimes it is, and in those cases we are able to "peak" into what your Firefox is spending so much CPU cycles on.
It's not perfect, but it's better than nothing :)