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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
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.