This is a special case of Firefox being nonresponsive while data transfers are taking place. Scroll keys don't work, either, but are stored in the buffer and give unexpected results when the transfer ends, or pauses. Clicking a link also gives unexpected results when the page is jumping around because of incorrect rendering. I have had to start Task Manager and close Firefox to get control of my computer back. No answers in three days. Well, we are an active discussion group, aren't we?

Ed.Ferris said

Sometimes closing the tab does not work, either, and my modem shows traffic even after Firefox is closed.

While Firefox has closed, the requested data is still coming in.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Separate Issue: Update your Flash Player Note: Windows users should download the active_x for Internet Explorer. and the plugin for Plugin-based browsers (like Firefox).

Note: Windows 8 and Windows 10 have built-in flash players and Adobe will cause a conflict. Install the plugin only. Not the active_x.

Flash Player Version: 22.0.0.209 Flash Player (Linux) Version 11.2.202.632

https://get.adobe.com/flashplayer/ Direct link scans current system and browser Note: Other software is offered in the download.

https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, Win IE . . . .) Note: Other software is offered in the download.

See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

It's difficult to replicate the problem, because I don't know in advance which webpages are going to get stuck on a data transfer. On starting Firefox in Safe Mode, a page I often use does not finish loading after two minutes. The Esc key has no effect, but I can, as I always could with this page, go to a different page. Is there an option to "Hide webpage until completely loaded", like the Page Transition parameter in Internet Explorer"? Or "Don't show images that don't have placeholders" to keep the rendering from jumping around? Flash and hardware drivers have nothing to do with the problem of Firefox being unresponsive during a data transfer.

Start your Computer in safe mode with networking. Then start Firefox. Try Safe websites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Please answer the question: "How do I get Firefox to stop loading a webpage?" The problem is with Firefox, not with the operating system or hardware drivers. I have a poor Internet connection, which has high latency at times, but I should still be able to stop a data transfer. Why doesn't the Esc key stop all running processes?

Ed.Ferris said

Why doesn't the Esc key stop all running processes?

That is not its job. Look at the address box and look for the X Sometimes, the X changes into the reload (circle) icon when the page is loaded.

Ed.Ferris said

Please answer the question: "How do I get Firefox to stop loading a webpage?" The problem is with Firefox, not with the operating system or hardware drivers. I have a poor Internet connection, which has high latency at times, but I should still be able to stop a data transfer. Why doesn't the Esc key stop all running processes?