It's still possible in the new version of Firefox, which you are now using, to stop a download while in progress. There is a red button on the right side of the Downloads window corresponding to the file. Click that and it will cancel the process.

Here is the link for more information about the Where to find and manage downloaded files in Firefox

Well, the download window will help if I want to stop a file download. What I want to do is stop the download of a web page, which can be VERY slow on dialup, which is all I can get here in the boonies....

That is stopped by using the button that by default is now on the end of the location bar, and is a stop button whilst a page is downloading and a reload button once it has downloaded.

You could also consider other options that may help you. For instance does a web page have a mobile option you may use. (could even change user account string). Consider turning off flash and javascript for instance. Maybe the noscript add on would help.