Firebug
FireBug is a new tool that aids with debugging Javascript, DHTML, and Ajax. It is like a combination of the Javascript Console, DOM Inspector, and a command line Javascript interpreter.
Regular Expressions Tester
Testing Tool for regular expressions with color highlighting
lori (Life-of-request info)
For a given page,
* track how long it takes to see the first byte from remote server (time-to-first-byte (TTFB)).
* track how long it takes to complete displaying the page (time-to-completion (TTC)
* track the page size
* track number of sub-requests used to build the page
Html Validator
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing.
The details of the errors are seen when looking the HTML source of the page.
The extension is based on Tidy. Tidy, was originally developed by the Web Consortium W3C. And now extended and improved by a lot of persons. Tidy is embedded inside Mozilla/Firefox and makes the validation locally on your machine, without sending HTML to a third party server.
Console²
Console² replaces the JavaScript Console with what could be the next generation Error Console.
As of Firefox 1.5, the JavaScript Console is used not only for JavaScript errors, but for CSS errors as well. This extension takes this into account by providing a simple possibility to display errors by type.
Tamper Data
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters.
Trace and time http response/requests.
Security test web applications by modifying POST parameters.
Das sind so die besten FF-Plugins die ich zu dem Thema gefunden habe, bzw. deren Einsatz ich persönlich sinnvoll finde. 
Teilweise etwas redundant, aber ich finde, sie ergänzen sich ganz gut.
Gibt noch zig andere Plugins in diesem Bereich, vielleicht hat ja jemand noch ein paar andere gute Vorschläge?