User Agent: Mozilla/5.0 (Windows NT 6.1; rv:46.0) Gecko/20100101 Firefox/51.0 Build ID: 20170125094131 Steps to reproduce: installed 51 and 52 Actual results: fonts are badly readable since the upgrade from 50 to 51 see attached screendump

Hans, please paste the graphics section of Thunderbird | help | troubleshooting

Graphics GPU #1 Description NVIDIA NVS 4200M Vendor ID 0x10de Device ID 0x1057 RAM Unknown Drivers nv4_disp Driver Version 6.14.13.900 Driver Date 10-28-2013 Features WebGL Renderer Google Inc. -- ANGLE (NVIDIA NVS 4200M Direct3D9 vs_3_0 ps_3_0) -- OpenGL ES 2.0 (ANGLE 2.1.0.2a250c8a0e15) AzureCanvasBackend skia AzureCanvasAccelerated 0 AzureFallbackCanvasBackend cairo AzureContentBackend skia

This is a know side effect of bug 1007702 and bug 1299903. Basically you don't like the new Skia font renderer, which is used when HWA is switched off. Neither do I. To change back to cairo, either switch HWA (hardware acceleration) on Tools > Options > Advanced, General or with HWA switched off set preference gfx.content.azure.backends to direct2d1.1,cairo (default is direct2d1.1,skia,cairo). The new default behaviour using skia is pretty much like Google Chrome. Check attachment 8786999 [details], left (good) without skia, right (bad) with skia.