I have noticed that Firebug and some other addons I have browsed require that Firefox be installed and running for them to be installed.
That's a little inconvenient, given that my development machine is not connected to the internet. Is there a way to download and install the addins using a flash drive?
You could trick the program into thinking that you're connected to the Internet
To manually install a Firefox extension into Firefox, download the add-on to your computer. If the download option isn't available, right click on the extension install link and select "Save Link As..". Be sure not to change the file extension. A Firefox extension will usually be a .XPI file.
Next, go to the Firefox drop down menus at the top of the browser. Select File > Open File then select the addon.xpi file and click on Open. The Software Installation window will appear, click install, restart Firefox.