I like these “Best” lists that you find on blogs, so I thought I should at least start contributing my own. And to begin with, my own list of swizzy Firefox add-ons that will help the humble developer. They’re all Firefox 3 compatible and they’re all free.
In no particular order (other than, erm, alphabetical. Which is an order)…
With a click of an icon you can switch Firefox’s rendering engine to IE and back again. Useful for those quick cross-browser tests.
This scans the current web page and highlights each link it finds according to whether it was valid or not – a great way to quickly check link validity on a page.
Live HTTP headers
Allows you to see the header information sent by a web page – particularly handy for checking on compression, caching, cookies and the like.
This is a quick and simple measurement tool. Normally, I’d recommend standalone apps for this kind of thing so they’re not restricted to the browser, but this does this particular job so well, it’s still a requirement.
When it’s time to keep an eye on your SEO, this is invaluable. You type your chosen search terms into the RankChecker screen and it will display, for a number of popular search engines, where your site came in the results. Great for monitoring whether your SEO is working or not.
This does a single job but, god, does it well. It takes screen shots from the browser. You can either take an area, the current visible portion or the whole page (whether currently on screen or not). You can then save the results or stick it on your clipboard.
SEO For Firefox
This displays various SEO information about the site – PageRank, links, etc
This is a new one that I’ve come across when I had a specific requirement. Basically, this lets you split the Firefox screen into multiple “windows” into which you can load different pages. In my case I found this invaluable when comparing two pages side-by-side (an existing version of a website and a new, modified version).