A security update from Microsoft Windows Update automatic updates on March 9 caused my Windows Vista computer to have blurry or fuzzy fonts in all my browsers, FireFox, Google Chrome, Opera.
I had a similar issue a couple of years back with another Microsoft update so I knew to look at recent Microsoft Updates.
If you have this issue, you can Uninstall this update KB3140735. Then you need to block the update from being reapplied in Windows Update.
I am using old Windows Vista. I do not know if it causes the same issues for other Windows versions.
Here is a link to details of this update on Microsoft web site.
https://support.microsoft.com/en-us/kb/3140735
