I am using a compression library for my js and css files. However according to yslow the file that it generates is not gzipped but it should be. So I want to verify this myself but I don't know how.
How do I do this in firefox or firebug?
You can tell by looking at HTTP Response Headers - look for 'Content-Encoding: gzip'
You can probably tell by drilling into the Net tab in Firebug, but I always used to use the Web Developer Toolbar (a Firefox extension) for checking response headers. There is also a lesser-featured extension called Live HTTP Headers. https://addons.mozilla.org/en-US/firefox/addon/3829/
Alternatively, you can google for a website such as this, to check for you: