This would save enormous amounts of time, as Firebug is what I use to debug anyways.
I would just like to save the changes I've made to a remote or even local directory. Is this possible?
(bonus: can I do php development also, and not just html/javascript/css?)
There's currently no automatic way to do that.
In addition, this would only work for plain HTML, JS and CSS files. If there anything generated on server side, you'll need to update your server-side templates by hand anyway, because the client (Firefox with Firebug) won't know about the template. It only seed the generated code.
You can view the rendered source using something like the Web Devloper toolbar (ie after your edits) and save that BUT:
So not entirely useful.
PHP is server-side. The browser (and thus Firebug) don't get to see it so it's a outright no to your second question.
On the side: if you find Firebug that much simpler for wholesale editing, you're doing something wrong. Your editor is pants or you need another screen for more desktop space. Have a go at trying something new to spruce up your workflow.
You can save the changes you made in firebug for css and html by just copying it and pasting in your editor of choice. However inside the firebug itself there is no save function that i am aware of .
I will second the Web Developer tool, it is amazing tool for writing css and html as you can see the changes you make for css and html right in front of your eyes. Web developer tool it is almost an editor on its own build into the browser.
For php debug with firebug check out the FirePHP extention.
You can use scrapbook to save edited pages, which, however, only works most of the time. It saves all files locally based on the current DOM.