Attached is a better quality image of the issue.

Thanks

hi, this is probably caused by microsoft update kb3013455 which can cause font rendering issues on vista:

After you install security update 3013455, you may notice some text quality degradation in certain scenarios. The problem only occurs on systems that are running Windows Vista SP2 or Windows Server 2003 SP2. Microsoft is researching this problem and will post more information in this article when the information becomes available.

^{https://support.microsoft.com/kb/3013455}

one option may be to roll-back this particular update for now until there is a fix by microsoft or you could also look if there is a graphics driver update available on the website of the vendor (intel/amd/nvidia) which might also resolve this problem in some cases.

Thanks very much. I had a suspicion it was not only Firefox. I'll wait a while until Microsoft has a fix. If not I'll roll it back. Latest graphics driver update for my system is from two years ago, so I doubt that will fix it.

it is worth a try!