These are the top 10 Firefox plugins
1. Adblock Plus. If you have ever been annoyed by all those ads and banners on a site that often take longer to download than everything else on the page, install Adblock Plus and get rid of them. Right-click on a banner and choose "Adblock" from the context menu, and the banner won't be downloaded again.
2. Colorful Tabs. This simple add-on makes a strong colorful appeal. It sets each tab to a different color and makes them easy to distinguish while beautifying the overall appearance of the interface. After a long day of research when you have lots of browser windows open, this makes online page viewing easier on the eyes.
3. ColorZilla. ColorZilla puts an eyedropper icon in your status bar. Click it and you'll get a crosshair cursor. As you run this over a Web page, the RGB values of the pixel under the crosshair will display in the status bar, both as three separate values and as a hex value (e.g., R:255, G:255, B:255 | #FFFFFF). I use this all the time if I'm trying to match colors; i.e. a font color to an the primary background on an image, for example.
4. GMail Manager. This Gmail notifier is great if you have multiple Gmail accounts. It allows you to receive new mail notifications along with viewing account details including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets.
5. MeasureIt. After installing this extension, you'll have a small ruler icon on the left side of your status bar. When you click on it, your browser window will fade out a little, and you'll have a crosshair cursor. Drag the cursor over a section of the screen that you want to measure. Next to the box is its height and width, measured in pixels. I use this all the time when trying to measure the size of images. When you're finished, just hit the Escape key to turn it off and return to normal viewing of the page.
6. Quirk Search Status. Search Status allows you to see how any web site you visit is performing. When you land on a page, SearchStatus lets you view its Google PageRank, Google Category, Alexa popularity ranking, Compete.com ranking, SEOmoz Linkscape mozRank, Alexa incoming links, Alexa related links and backward links from Google, Yahoo! and MSN. This combined search-related information means you can view not only the link importance of a site (according to Google and Linkscape), but also its traffic importance (according to Alexa and Compete), so providing a balanced view of site efficacy. I use this all the time to determine whether a site has enough traffic to warrant accepting a joint venture proposal.
7. Scrapbook. ScrapBook helps you to save Web pages and easily manage your saved collections. Major features are: saving web pages or snippets of a page, saving a web site, organizing the collection in the same way you do bookmarks, full text search and quick filtering search of the collection, and editing of your collected pages.
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8. Session Manager. Session Manager helps you manage your Firefox tabs. If you visit the same sites every day, all you need do is open all the sites in separate tabs and/or windows, and then use Session Manager to save the session with a distinct name. Then, you simply go to Tools > Session Manager, pick your session, and all the windows and tabs open up just as you saved them. And, Session Manager tracks your sessions as you surf, and if Firefox (or your system) crashes, you can recover the selection of tabs you had open when it crashed.
9. Tabs Open Relative. Tabs Open Relative makes all new tabs open to the right of the current tab, rather than at the far right of the tab bar. This reduced a huge annoyance I had with how the Firefox browser worked.
10. XMarks. XMarks (formerly Foxmarks) provides seamless bookmark synchronization between your computers and browsers via their synchronization server. Your bookmark (and optionally password) data is securely stored and backed up on their servers and is available online, as well. After you install the add-on, click on the notification to set up Xmarks and start backing up and synchronizing your bookmarks. Install Xmarks on each computer you use, and it seamlessly integrates with your web browser and keeps your bookmarks safely backed up and in sync across all of your computers. Secure Password Sync is an optional Xmarks feature.