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How to have max width on tab
(jemmy.jeena.net)
A place to discuss the news and latest developments on the open-source browser Firefox
That ain't normal. You can certainly customize Firefox to behave that way with custom userChrome.css file, but you would have to do that on purpose.
I suppose it's also possible that if Firefox is installed via your package manager then they might do some customization to it. I've seen some linux distros package these sorts of custom Firefox builds that apply various changes to official Mozilla builds.