After upgrading to FF 18, my Gmail started to look blurry. I'm using Windows 7 and I have Cleartype on and I've tuned it well.
All other sites look alright. It's only FF 18 that started to look weird.
Do you have smoothing turned on as well?
Have you reset Firefox? Refresh Firefox - reset add-ons and settings
It was false. I set it to true and restarted. Still the same. It's only Gmail and Google Drive that looks weird.
Can you attach a screenshot?
Use a compressed image type like PNG or JPG to save the screenshot.
Try to disable hardware acceleration in Firefox.
Try to set the gfx.content.azure.enabled pref to false or if this didn't help disable Direct2D by setting the gfx.direct2d.disabled pref to true on the about:config page and leave hardware acceleration otherwise enabled.
Screenshots of my Gmail and another website attached for comparison.
Notice that Gmail text looks blurred - as if Cleartype is not working well?
Both were taken while viewing the sites using FF 18.
Just checked. hardware acceleration is on.
Does it help if you select another font to display the text?
You can use this extension to see which fonts are used for text that is selected (right-click context menu: Show fonts in selection).
You can do a font test to see if you can identify corrupted font(s).
You can try different default fonts and temporarily disable website fonts to test the selected default font.
Try to reset the page zoom on pages that cause problems.
You can remove all stored data from a specific domain via "Forget About This Site" in the right-click context menu of an history entry (Show All History or History sidebar) or via the aboutermissions page.
Using "Forget About This Site" will remove everything like bookmarks, cookies, passwords, cache, history, and exceptions from that domain, so be cautious and if you have a password or other data from that domain that you do not want to lose then make a note of those passwords and bookmarks.
You can't recover from that "forget" unless you have a backup of the affected files.
It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.