Does this proxy add an extension to Firefox, or is it purely external, at the system level?

If it uses an extension, could you check why the extension is disabled? You can find that information on the Add-ons page, either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then find the extension on the right side at the bottom and click its "More" link. Firefox should give the explanation there as to why there is a problem.

If the page says it is not verified and gives a link to this article "Add-on signing in Firefox", you could make a temporary exception to the signing requirement. There's no way to make an exception only for one extension, so keep an eye out for unwanted extensions that might take advantage of the exception.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste xpin and pause while the list is filtered

(3) Double-click the xpinstall.signatures.required preference to switch it from true to false -- note that this is for Firefox 43-45 only

See also:

• FoxyProxy Standard: https://addons.mozilla.org/firefox/addon/foxyproxy-standard/
• AutoProxy: https://addons.mozilla.org/firefox/addon/autoproxy/
• QuickProxy: https://addons.mozilla.org/firefox/addon/quickproxy/

Thanks both of you. I followed the instructions from jscher. I still do not get through to blocked sites such as youtube.

That may now be Astrill's problem, although I did get a friend to send me the very latest version. My friends here who also use Astrill vpn can use it. But they are Windows users and I am a Linux man.

As you may or may not know, because of the big meeting in Beijing, China has been rigorously blocking everything, especially vpn services. Astrill completely stopped working.

Even before that, I had this trouble with Firefox not allowing Astrill Proxy Switcher. I wrote to Astrill. They gave me some instructions, which I followed, to no avail. This is really Astrill's problem. They must do whatever it is to make Astrill Firefox compatible.