What proxy extension are you using?

Did you check its Add-ons page to see if problems are reported?

You can open the Browser Console (3-bar Menu button or Tools > Web Developer) and check if there are error messages about this extension.

• https://developer.mozilla.org/Tools/Browser_Console

It is not the proxy that's the problem because the on/off button is still visible and the proxy is still working fine in Chrome. I can't turn the proxy on in Firefox (preferred browser) any more because the button/icon is no longer there in the top right of the window. My adblocker icon and WebRTC pops up when I open Firefox, but not the proxy button, which was there, and had been for months up until a couple of days ago.

Hi, open the 3-bar menu > Customize, and if you can see the button, simply drag it to a toolbar and the location of your choice, then Exit Customize. If the button isn't there, click on Restore Defaults.

More here: Customize Firefox controls, buttons and toolbars

If that doesn't help, try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem. Thank you!

Thank you Scribe for all your suggestions, which were really easy for me to follow. Alas I tried them all without success, so I have now contacted the proxy developer's support page in the hope that they can shed some light on why the button just suddenly disappeared. If and when the problem is solved I'll definitely let you know by posting on this page.

Thank you again for your assistance.

As mentioned the on/off icon button to one extension suddenly disappeared from the toolbar. After trying the suggestions offered on this page in an attempt to restore the button to the toolbar I experimented by installing another extension which also required a button in the toolbar to change the status of the extension, but the icon button did not appear in the toolbar for that extension either.

Solved this myself. Addon icon buttons would only appear in the toolbar after I unchecked "Always use private browsing mode" in the privacy settings.