This is related to smoother font rendering in Firefox (ex. 41.0.1) and this question from Mozilla Support "How to make fonts in firefox look prettier? For example like in sleipnir"
I GOT IT WORKING!
I have 64-bit Win7 with decent hardware and cpu, and I also have gdi++
Had the same problem, tried every "gfx" option in about:config to no avail until I turned off "Use hardware acceleration when available" in the Advanced options. This idea comes from "How to Disable Hardware Acceleration (GPU Rendering) in Mozilla Firefox and Thunderbird to Fix Font Problem?" article
Just an example how it looks now:
Missing the letter "l"
"I found a fix for this myself a while ago but unfortunately I don't think anyone saw it since the system of conveying information here seems to be a bit archaic.
Go to your gdipp folder and open gdipp_setting.xml
Find where it says renderer, and it should be at 10. Set it to 11 and the missing "l" problem should be fixed. "