People seem to like posting their favorite plugins and software these days, so I thought I could add my little list of useful stuff here, too. I’m open to suggestions for other good extensions, of course.
Here’s my list of Firefox AddOns I am using actively:

• Adblock Plus
• Firebug
  A powerful debugging aid for web developers. Comes with e.g. a DOM explorer and a JavaScript debugger.
• Web Developer Toolbar
  A toolbar which offers many useful functions for web developers. In particular, I like outlining of block elements or enabling read-only form fields.
• FoxyProxy
  Manage proxy configurations and use them depending on URL patterns etc.
• ImageZoom
  Zoom images with RMB and mouse wheel.
• Foxmarks
  Synchronizes bookmarks. I found it particularly useful to define synchronization profiles so that I can separate work bookmarks from private stuff.
• Tabinta
  Ever tried to enter a TAB in textarea controls? Tabinta allows you to use TAB there.
• Flagfox
  Shows the country flag of the server’s origin in the address bar.
• Split Browser
  Splits the browser window into multiple areas.
• Nuke Anything Enhanced
  Allows to remove any DOM object under the mouse. Useful e.g. to get rid of layer ads that Adblock Plus doesn’t catch.

I also found the following OS X software to be useful:

• On the Job
  I bought this one to do my expense tracking and invoicing when I do freelance work. It can be tested freely for 20 days and I really liked it from the beginning, but I got convinced to buy when it offered me another 10 days trial after the first period was over – wasn’t that nice?
• Cyberduck
  Great FTP client.
• TextWrangler
  Powerful text-editor with FTP-support.
• The Unarchiver
  Unarchiver which supports plenty of file formats. Good replacement for the native ZIP tool which is pretty limited.
• LabTick
  A little tool which keeps the keyboard light on all the time, independent of the ambient light.
• Little Snitch
  Firewall which monitors outgoing traffic and keeps programs from “phoning home” if you don’t want them to.
• ChMox
  Tool to read Windows CHM files.
• 0xED
  Small and fast hex-editor.