1. SEOpen ( https://addons.mozilla.org/firefox/570/ ): this is one of the best and most used Firefox seo plugins, and has many tools that make it a favorite of mine. So let's see what it can do:

* in Google it can check for: backlinks, indexed pages, related pages, cache and PageRank.

* in Yahoo and MSN: backlinks and indexed pages.

* for Alexa: overview, traffic details, backlinks and related sites.

* keyword density of the most used words on the page.

* DMOZ checker: see if the site is included in the directory.

* check for robots.txt.

* validator to check your page for errors (it uses the validator from www.w3.org).

There are other things, but these are the most important.

2. SEO Links

( http://www.webmasterbrain.com/seo-tools/firefox-extensions/seo-links/ ) is another extension I use everyday. When the extension si active and you hover over a link a tooltip will appear that will show you Yahoo, MSN, and Google link popularity and ranking data for the URL and anchor text pair. When you're simply browsing this can be annoying, but you can simply deactivate it with a right-click!

This could be very easily used to check your search engine rankings for a certain page, and here is how: write up a list with all the keywords and keyphrases that you want to rank for, and then link those words or phrases to the web pages associated with them!

Now every time you want to check your ranks activate the SEO Links extension and hover over the links -- you will see where your pages are in Google, Yahoo and MSN.

3. Don't waste anymore of your precious screen real estate just to find out what your PageRank and AlexaRank is! SearchStatus ( https://addons.mozilla.org/firefox/321/ ) can sit anywhere you want (I keep it in the status bar) and you'll receive the precious info at a glance. But this little tool can do a lot more than that:

* it shows what metatags a site has

* checks for backlinks and indexed pages in Google, Yahoo and MSN

* it shows the robots.txt file

* keyword density report function

* and best of all: the highlight "nofollow" links will show you what links will be followed by search engine spiders -- very useful when you want to submit your site to a directory and don't know if it will matter from a seo perspective.

So in case you're not using Firefox, go get it! It could be your best friend when it comes to search engine optimization!