Frequently when browsing the web, and increasingly with all the active content out there, websites will often load/reload more content without the browser considering it to be loading. Thus Firefox's smart button that alternates between reload and stop depending on context will often be displaying the "reload" option, when the "stop" is what I really want. Ideally, this would be a "stop all activity/connections" behavior that interrupts all loading and other activity.
Ideally, this fixed-purpose stop button would be an optional one that people could choose to put into their toolbar, but not be forced to do so.
The extension store does have an extension that adds a stand alone stop button... But I have to give that extension access to my tabs in order to work, and it states that it isn't monitored or verified by Mozilla, so "I should be sure that I trust it." I have no way of knowing if I should trust it. I don't know where the source code is, or even if I did, no way to know if the source code is actually what the extension is, and probably couldn't understand the code anyway. Likewise, I have no idea who the developer is. In short, there's no way for a regular user to know if they can trust it; and it's such a basic feature of a web browser that users should be able to have access to it without needing to install extensions anyway.