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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
Maybe because the software is designed to make that very practical and smooth. You also might point to hardware limitations, should you have a machine that doesn't have a lot of RAM, or perhaps you might point to simplicity, and that you don't want to have a cluttered taskbar.
But it's kind of ironic that you would ask why not leave software open on a post where the problem was specifically mentioned as one that is solved by closing the software.
So perhaps another anecdote is in order. I currently running three instances of Firefox (different profiles) on a low-end Celeron laptop. I don't usually shut them except sometimes by mistake. What I do do is close tabs, if only for simplicity's sake (because idle tabs are unloaded from memory anyway). I'm experiencing no sluggishness issues.