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If you have made changes to Advanced font size settings like increasing the minimum/default font size then try the Default font size setting "16" and the Minimum font size setting "none" in case the current setting is causing problems.

• Tools > Options > Content : Fonts & Colors > Advanced > Minimum Font Size (none)

Make sure that you allow pages to choose their own fonts.

• Tools > Options > Content : Fonts & Colors > Advanced: [X] "Allow pages to choose their own fonts, instead of my selections above"

It is better not to increase the minimum font size, but to use an extension to set the default page zoom to prevent issues with text not being displayed properly.

@cor-el your solution is not working in my case..btw I had not changed any font settings and it is working fine with older versions of firefox as well as firefox versions in linux...and it is also working in windows 10 but showing problem with windows 8.1 latest version of firefox...