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For me it is:
1.
DownThemAll!(or just DtA) is a powerful yet easy-to-use Mozilla Firefox extension that adds new advanced downloading capabilities to your browser.
DtA lets you download in just one click all the links or images contained in a webpage; to refine your preferences you can use fully customizable filters, and select just the kind of files you really want to download.
2.
MR Tech's Local InstallIt provides the tools needed to install and manage extensions and themes locally. To do this the extension provides multi-extension installation support, hacking capabilities to the Extension/Theme manager windows, features to find and troubleshoot Extensions/Themes Build, GUID and Profile information.
3.
Translate PageTranslate Page passes the current page to Google's Translation Service and was made upon request to directly mimmick a feature of the Mozilla suite within Firefox.
4.
ScreenGrabSaves webpages as images by scrolling around and stitching the result together. In particular it also will save just a frame as an image.
5.
HTML VALIDATOR (based on Tidy)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.
There are many more good ones that I use, but this is what I use the most!