Actually I don't think it does for values. I just tried to 'change' the value of a font CSS property on Hacker News from Vedrana to something else and it didn't offer any font names.
What do you expect? Should it show every possible font you can type into the property? Just the fonts installed on your machine that other people may not have?
Just any property that has a preset number of values show up as options as you type. The difference is that Chrome shows a dropdown list while with Firefox you have to sort of remember the values as it does an autocomplete. I see no reason Firefox couldn't offer the dropdown since it clearly knows the possible values.
By the way, I tried the same font thing in Chrome and it responded the same way, no font list. You can see a similar behavior with color, for the value it shows possible colors by name. Start your color with # and suddenly there's no options. If there are too many possibilities to list it doesn't list anything.