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Firefox becomes slow after a couple of weeks open
(bugzilla.mozilla.org)
A place to discuss the news and latest developments on the open-source browser Firefox
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.