Firefox Extension Help
 

I have installed the Header Monitor, which is supposed to display HTTP headers in the status bar, but it isn't doing so. The options panel doesn't yield any clues as to how to make it work.

Also, I downloaded the Web developer extension. I was not given the option to install it, so I downloaded the XPI file to my desktop. What am I supposed to do with it now?

Figured out the Header Monitor thing. Still no clue what to do with this XPI file.

Figured out the other thing too. The extension wasn't Firefox compatible, which may explain why it didn't offer to install it in the first place.

You can install an XPI file (Firefox/Mozilla extension), by selecting the "File" menu and then selecting "Open File...", browsing for the XPI in question and clicking the "Open" button.

The program will open the file, realize it is an Firefox/Mozilla extension file, and go through the installation process.

I use this method to install XPI files quite often as I have found that the installation-from-web-just-by-clicking-on-the-XPI-file-link-on-the-webpage method to be flaky at best.