Relative term, and totally subjective. I have an rMBP, and use Firefox (as well as InDesign) all the time, and it's not horrible. If your entire screen looked that way, you wouldn't even notice.
Left is Firefox, right is Chrome. Nothing special done to either. MBP is at the factory resolution. Very ironically, the screenshot at half-size looks better on my 24" Windows display than it does on the actual MBP does, presumably as Chrome is doing some resampling to downsize it. Look at it at full zoom to see what I mean.
I'm doing active web development on a number of browsers across a number of devices (Macs, Windows, iOS, Android), and Firefox on the Mac is easily the worst of the lot, by a rather large margin. I would easily notice if the whole screen looked like it - text is blurry and awfully aliased, to the point that it's jarring to read. I spend most of my time on Windows, which by most accounts has less beautiful text rendering than OS X, and switching over to Firefox on the rMBP feels just awful.
Given that I spend 99% of my day looking at a 94 DPI screen, I don't think it's too much to ask that the same application on my 220 DPI display not look like I'm playing Minecraft.
The text is fuzzy and blurry. You may not see it if you don't zoom in (to 100%), because your browser will apply crappy resizing filters to the whole image, so both browsers will have the same look.
Direct comparison of some of the more egregious examples in Firefox (Top) vs Chrome (bottom). Look at the tab title, the text in the dark grey nav bar, the aliasing on the back button, the aliasing on the text, the aliasing on the edge of the "O" in the Google logo, and the aliasing of the text under the buttons.
It's more obvious when zoomed in and I can see your point. But do you not experience the first image, the zoomed out version? In that one I can barely differentiate.
It's pixel-doubled like every other non-native-Retina app on your Mac. Why is it Firefox's problem if Apple didn't bother to go to the trouble of making non-Retina applications readable on a Retina display? The fact that they don't even bother to do filtering when pixel doubling is probably a big contributor to readability issues (note that Vista and W7 do filtering when doing DPI adjustment, likely for this exact reason), not to mention the fact that it appears to be doing subpixel antialiasing when there are no subpixels to address (WTF, Apple?)
> Why is it Firefox's problem if Apple didn't bother to go to the trouble of making non-Retina applications readable on a Retina display?
I'm not sure what they could have done beyond making sure Cocoa's text-rendering routines produce the right "retina" output. Which they do. Firefox (and Chrome) do their own text rendering and rasterizing, as far as OSX can see it they give it a big image to display. Not much OSX can do with that.
