If you are really interested in getting the best from your Google Reader, here is a list of verified downloadable add-ons (extensions) that will work smoothly on Google Reader with Firefox. :
Littlefox – This add-on is used for optimum screen space. It creates a navigation bar that can easily be expanded and collapsed, resulting in plenty of space for reading the feeds.
Better GReader – You can preview a web page inline. Header and sidebar can also be collapsed for more reading area. GReader also provides Mac OS X like skin.
Google Gears – It is very nice. It allows you to run Google Reader offline. With Google Gears, you can download your feeds onto your PC and then access GReader offline. It is very efficient, the work you do on GReader while being offline will be synchronized with your GReader account when you next time go online.
Desktop Notifiers - Notifier alerts you of new feeds. It is very good for keeping yourself up-to-date every second. Alternatively, use Google Reader Button (Also requires Google Toolbar)
Read by Mouse – Very cool add-on that lets you navigate items using mouse motions. The left and right mouse buttons will function as pointers to the next and previous feed items. If there is a middle button on your mouse, it can be configured to start the “share”, “star” or “open in a new tab” functionalities.
Google Reader Custom Search – Use this to search your feeds inside Google Reader.
Smart Subscribe – It helps you know whether the web-page you are on has already been listed among your subscriptions or not.
Google Reader + Del.icio.us – If you are frequent visitor to Delicious.com and like to bookmark items to Del.icio.us, this add-on is for you.
Google Reader Integration – This is another cool add-on that integrates Google Reader box into GMail sidebar.
Optimized – This is for maximizing reading area. It removes rarely used items .It is good when you have too many items to go through, this gives more friendly view of the contend area.
Mac OS X Theme This script makes GReader to look like any other Mac-styled app. It is great for Mac lovers.
Colorful List View – Use this if you want colorize item headers in your Google Reader.
FeedShake This extension will allow you to merge, filter and sort multiple RSS feeds. An alternative to this is Yahoo Pipes.
Hope you found these add-ons perfect for your use.
These extensions are not always of direct interest to Google Reader users but If you use Google Reader regularly for reading web-feeds then these cool extensions will definitely benefit you a lot and save your time, too.
Please do visit the following Google Reader article as well, which explains how Google Reader can be further enhanced - Themes Change the Look of Google Reader