Stop button: What does it do?
If I start opening a hundreds-megabyte PDF, or a JS-heavy website, or a server just do not respond at all, I expect that when I'll press "Stop" button browser would... emm... stop: stop trying to render the rest of the page, stop trying to establish the connection... But notning happens. I don't see any reactions to Stop button. The only way to stop the download is to close the tab (I'm not sure, I hope it stops). By the way, it is especially "nice" when the program closes together with the closing of the last (as suddenly will be realized later) tab.
So, there is the "Stop/Reload" button. Does it do something like stop something? Or it is purely decorative/irritative? Or I just press it wrong way?
Regarding Stop, someone else may know more technical details. My assumption was that it cancels pending network requests and doesn't send any new ones, and may interrupt some layout functions, but it probably lets others run to completion (or if they are nonresponsive, then it can't do anything about that).
The only way to stop the download is to close the tab (I'm not sure, I hope it stops). By the way, it is especially "nice" when the program closes together with the closing of the last (as suddenly will be realized later) tab.
You can set Firefox to open a new tab when closing the current tab would close the current window:
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
(2) In the search box in the page, type or paste LASTTAB and pause while the list is filtered
(3) Double-click the browser.tabs.closeWindowWithLastTab preference to switch the value from true to false
More info on about:config: Configuration Editor for Firefox.