To be honest, I did not experience it first hand myself, because I've read this behaviour from many other people.
What I know is that I have more than 300 tabs in Firefox currently, both on my home machine and my work machine, and I don't have any problems with it.
Edit: And I would add that my task manager is also not polluted with hundreds of chrome processes.
There are extensions or settings in most browsers that will lazy-load tabs for you. No transfer or rendering occurs until you open the tab.
That aside, what could possibly require opening 300 tabs? That must be an absolute productivity killer... Why not use a bookmark manager or "read later" service or something?
I don't know what would need 300, but if I'm in the middle of working on a project, I might [open link in new tab] dozens of tutorials, docs and other resources as I gradually become more familiar with the domain and the gaps in my knowledge.
Sure, they should get pruned to bookmarks, but you can't argue if a person's method of staying in the "zone" works for them, I suppose.
What I know is that I have more than 300 tabs in Firefox currently, both on my home machine and my work machine, and I don't have any problems with it.
Edit: And I would add that my task manager is also not polluted with hundreds of chrome processes.