In Firefox Safe Mode hardware acceleration is disabled, so it seems that you have a problem in this case.

You can try this extension:

• Anti-Aliasing Tuner: https://addons.mozilla.org/firefox/addon/anti-aliasing-tuner/

Hm ok, but if this was the problem, I should be able to reproduce it by disabling/enabling hardware acceleration manually, or not? But like I've explained above, I have already done that.

You currently have a 64 bit Firefox version.

Is the other computer using a 64 bit version as well?

Note that you need to close and restart Firefox after toggling hardware acceleration or modifying related prefs manually.

See also:

• http://mxr.mozilla.org/mozilla-release/source/gfx/thebes/gfxPrefs.h

I actually tested all of this with 32 bit versions. And yes, Firefox has been restarted after doing the manual changes.

This can be a problem with the font that is used to display the text.

You can do a font test to see if you can identify font(s) that aren't working properly.

• http://browserspy.dk/fonts-flash.php?detail=1

You can try different default fonts and temporarily disable website fonts to test the selected default font.

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

Make sure to restore the default setting and allow pages to choose their own fonts when you are done with testing.

I appreciate your help, but you are also giving me the feeling that you didn't fully process the information given. If it was the font, how could it be broken in three browsers as described, but work in the fourth? By logic, that is impossible.

When you switch to Safe Mode and hardware acceleration is disabled then it is possible that Firefox switches to another font for some reason. This can also depend on zooming directly or device dependent (layout.css.devPixelsPerPx)

You can right-click and select "Inspect Element" to open the Inspector (Firefox menu button or Tools > Web Developer) with this element selected. You can check the font used for selected text in the Font tab in the right pane of the Inspector.

• https://developer.mozilla.org/Tools/Page_Inspector

When DirectWrite rendering is used rather than GDI rendering, font families are grouped differently
How a browser reacts here will depend on which font subsystem it's using.
Windows has two different font subsystems: GDI and DirectWrite.

First of all, my apologies if I came across as curt yesterday, it was already fairly late in my part of the world. Having said that, I am very certain that Firefox did not switch to a different font.

Generally, I believe my best bet is to manually recreate the status where "Safe Mode" makes the font look ugly. If I can get there, I will also be able to dig down to what actually made the difference. At this point, however, it seems like "Safe Mode" does things beyond what I can recreate in the Firefox configuration, and this is my problem - they don't seem to be documented.

If you think I should take a different route, please let me know so we can both be on the same page.

I've been able to identify the problem - more or less. I'm posting my findings here so others will hopefully save some time. I've actually had this on three (!) computers - all are older models, but nonetheless.

Turns out that the problem was with direct2d - if it is disabled, font smoothing won't work either.

How to find out if you're affected: enter about:support in firefox, then scroll down. In the graphics section, there is a line starting with "Direct2D activated" - if it says that Direct2D has been blocked because of issues with your graphics driver, then this post is for you.

For your information, take note of the type of graphics card you have (if you don't know, you can find that info on the about:support page). Check out the following page to see if your card is listed (most likely, it is): https://wiki.mozilla.org/Blocklisting/Blocked_Graphics_Drivers

That page will also give you a hint what to do: If you graphics driver is outdated, just update it, and things might work again.

If you're not as lucky, updating might not do the trick, or you had an up-2-date driver to begin with. In that case, enter about:config in Firefox and search the configuration for "direct2d". If that does NOT bring up a blacklist entry, then try setting "gfx.direct2d.force-enabled" to "true", that should do the trick.

If you see an entry with the name "gfx.blacklist.direct2d" things will be even trickier. I've tried resetting that on one computer, but it didn't work - after restarting Firefox, it was back to the previous value. Searching for "sanity" and "driver-init" will bring up more entries that are likely related, but it seems like all of them are reset on restart.

According to [https://support.mozilla.org/en-US/que.../938832|this] thread, setting "layers.acceleration.force-enabled" to "true" might help, but I'm afraid I cannot confirm that.

So at this point, your problem is either solved or at least you're closer to a solution. Any other findings - please post them here.