You could use Troubleshooting Information from the Help menu to find the internal name for the extension and simply remove it from the extensions directory in the profile. After that, it would be a good idea to make sure that there are no settings from it left over in about:config. If there are, you can reset them.

Hi, thanks for helping me. The Firefox troubleshooting page (about:support) shows the extension as "Avast Online Security" but doesn't provide a path to where it's installed. There's a button on this page to open the profile folder, which contains an "extensions" folder with a couple .xpi files, but the Avast extension is one one of them. Still stuck...

OK, figured it out. There is a file called "extensions.json" in the profile folder. I searched that for Avast and found this path:

/Users/david/Library/Application Support/Mozilla/Extensions/{ec8030f7-c20a-464f-9b0e-13a3a9e97384}/wrc@avast.com

The plugin was installed at this path, so removed the files, re-started Firefox and...plug-in gone!!

Extensions and plugins are not the same thing. A plugin is called only if needed, and is a separate application without an interface, which the client application must supply. An extension makes itself part of the application.

dbbyres said

Hi, I've removed Avast from my Mac but the Avast Online Security (version 10.0.2502.149) still appears as an extension. The Avast uninstall app does not remove. There is no remove option shown for this extension. Anyone know how to actually remove it?

there is one other way more how to remove it to what others suggested, Avast Browser Cleaner, unmark exclude adds with high rating & it shows u all Adds, for one this folder direction does not exist in my PC, i guess in my PC it is in different location, Windows 8.1, i removed it with Avast browser cleaner

This add is pointless its only shows u what are suspicious sites & its paste of users votes, only people who get their first PC mite need it, it helps to lower the threat of user doing something dumb, unless the user ignores Avast add-one, also i believe all here know 1 fact that even knowing suspicious sites won't make browsing secure, u can get attacked from trusted sites to, it has happened to me to, luckily Avast has blocked all, it does not happen often, that's another reason its pointless add, it just visual, it has no security in it, i wrote it just in case some1 who does not know much about this add happens to visit this page