I'm pretty sure everyone already knows about Firebug for Firefox, which allows you to inspect code, but the plugins are what makes it shine.
Firefinder - This saves so much time in identifying CSS selectors, such as if you want to install a Wordpress plugin on a non-default theme.
CSS Usage - If you use any type of bootstrap framework for development, this can help you shave space from your stylesheets.
YSlow - Since Google probably takes into account how fast pages load, optimize them.
There are other ones, but these are probably the most useful in my opinion.
Firefinder - This saves so much time in identifying CSS selectors, such as if you want to install a Wordpress plugin on a non-default theme.
CSS Usage - If you use any type of bootstrap framework for development, this can help you shave space from your stylesheets.
YSlow - Since Google probably takes into account how fast pages load, optimize them.
There are other ones, but these are probably the most useful in my opinion.