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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
You can see the worst offenders in firefox by using the hamburger menu then more tools and Task manager. You can sort by ram. YouTube likes to hold gigs of ram for some videos. Close the biggest offenders and you'll get back close to normal speed.
In my experience this doesn't matter. Firefox just slows down if it's been open for long, regardless for how long the tab has been open for. Even if you unload all active tabs and open a new one, that new tab will still be significantly slower than it would be if you restarted the entire browser.
It's some kind of slow resource leak somewhere.
Ding ding ding, the only good reply in this thread.
The symptoms described by OP smell like good old memory exhaustion.