I download Firefox plugin for myibay but can't figure out how to install it. I'm getting unknown file or some similar error message.
Please help

I would suggest to re-download the file, seems like your download process was not right or file is corrupted.
Make sure that you use Firefox browser to download, it should detect it and install it automatically while downloading.

Here is a direct link from the official Firefox add-ons website:
https://addons.mozilla.org/en-US/firefox/addon/5571
If you have an old version of Firefox browser, then you need to update it to 2.0 or 3.0 before downloading this add-on.

Please feel free to ask more questions here if something goes wrong.