Must have "Developer-Assistance" Modules and FireFox Add-ons?

a) I always install the following modules on every site: coder, devel, drush, admin_menu, masquerade, cvs_deploy, jquery_update, & cache_disable.

b) firebug & the web developer toolbar. Not really firefox specific, but i use a number of bookmarklets from the following page: https://www.squarefree.com/bookmarklets/webdevel.html. I use the "test styles" one every single day.

I install the following on every site as a base:
* backup and migrate
* dhtml_menu
* css gzip
* parallel (just started adding this recently

There are others I use very regularly as well, but these are the ones I definitely install.

For browser add-ons, as everyone else has said, firebug is the best, hands down.

I also use a couple of other developer firefox addons as well:
* colorzilla (allows for any pixel in the browser to be clicked, giving you it's various color values (hex, RGB, whatever)
* HTML Validator - it puts an icon in the bottom of your browser that gives you a green check for valid code, and a red X for invalid code. If you are a person for whom valid code is important, this is a great module. When the icon is double clicked, it pops up the source code, with a listing of all errors that can be clicked to be taken right to the error.
* IE Tab - allows you to switch to the IE rendering engine within Firefox for whatever version of IE you have installed on your PC.
* Live HTTP Headers - gives a live view of all page headers (incoming and outgoing) when browsing the net. Good for when you are trying to set headers.