GlimmerBlocker is no longer maintained and should no longer be used as web sites have migrated from using plain http to using https (i.e. encrypted) which prohibits modification by a proxy unless you resort to install custom SSL certiticates on the client.
You should use a browser extension instead.
It was made as a hobby project during 2007-2008 by Peter Speck. It features both a custom http server and http client and a Mac OS X control panel.
Firefox doesn't automatically use the system-wide http proxy settings, so you have to configure it manually:
Select 'Preferences' from the Firefox application menu:
Select the 'Advanced' tab, and the 'Network' sub-tab:
Click the 'settings' button:
Select 'Use system proxy settings'
If you're still using Firefox 3.0 or earlier, you must select "Manual proxy configuration" and enter "127.0.0.1" and "8228" in the "HTTP Proxy" fields.