It's either you need more RAM or you must learn to use a tab group extension. Also, if it gets slow, just restart it.

Simple Tab Groups is a nice add-on.

My personal favourite is Sidebery. It has vertical tabs and easily navigatable via mouse wheel. You can even unload a tab. And has tons of customization options.

Try using a tab suspend extension, something like 'auto tab discard'. Firefox has one built-in, but it's not aggressive enough.

This is some S tier trolling.

Check the RAM usage of each tab. My Firefox is constantly open at work, albeit with anywhere from 1-10 tabs, and it never gets slow. Only time I restart it is when Firefox updates.

I ran into the same issue on my PC.

Dawg I had like ~35 tabs open and hadn't restarted my PC in over three weeks. Fucking Firefox was sucking back 80 gigs of RAM. 80 fucking gigs.

On the bright side all the tabs were still loaded when I clicked through them.

Adeptus mechanicus would be proud of this dude.