You can probably close this window via "Command + W".

Try to rename (or delete) the xulstore.json file in the Firefox profile folder.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thanks for the reply.

Closing it isn't the issue, I can close it by pressing <esc>. It then asks if I want to abort the downloads and if I say yes it closes.

What I want is to resize it to normal and then be able to resize the window. I'm sure I used to be able to do this.

I will try your suggestion on the .json file.

Isn't there a resize icon visible when you place the mouse pointer at the bottom right corner?

Can you open this window via the Options/Preferences button of the extension on the about:addons page?

There are no pointers or grab handles on any of the window edges or corners - believe me I tried!

No matter how I open this window (there are several ways) it is always coming up maximised at full screen size. All I can do is drag the title bar to move it down out of the way.

I tried another addon called something like "more content in tabs" - what that does is move the window content for this addon to a tab.

This would be fine except that every time I add another download (this addon is a download manager) it adds another tab and they all run in parallel and the downloads fail - I think it duplicates the downloads with the same name in each tab.

I just want to add, this addon window used to be fine before all of this.

I think it was after following the steps in this article on Firefox that caused all the problems:

http://www.laptopmag.com/articles/use-os-x-el-capitans-new-split-screen-mode

Renaming the .json file has fixed it. Many thanks!!!