Assume that I\'ve fired up Firefox with Firebug enabled on a random site (say slashdot.org). After loading the site I start editing the page using the Firebug > HTM
I think this is a good question, in short, firebug is not made to 'keep' your edits, so I would suggest finding a work flow where you are never too 'deep' into a page. One thing is to save the HTML but saving your edited CSS is even harder. Here are my solutions to your questions:
Click on HTML tag, click Edit. Copy and paste into another text file.
You cannot 'reload' firebug manipulated HTML. You can however copy the manipulated HTML after you are happy with your results (or unhappy). Either keep it on your clip board to copy it to a text file and then when you want to use it again you need to copy it back into the firebug edit console.