It's broken. You have to change something.

https://support.mozilla.org/en-US/questions/1198517#answer-1061078

http://forums.mozillazine.org/viewtopic.php?f=48&t=3037114

Thanks--I appreciate your reply. Unfortunately, I am reluctant to make changes myself - it is just too risky to download from unknown sites or to make changes to code. Do you think the add-on will be fixed (i.e., there will be a regular update to the add-on to fix it) or will it not be fixed now that there is no support?

The author of the add-on appears to be strangely quiet on this topic. Given his excellent track record of frequent updates, I am surprised and disappointed that there is no mention of the current situation nor any sign of an official fix coming along.

Do you not have anyone you can trust to look at the solutions offered, and choose then apply one of them for you?

The Config Editor tweak is probably the easiest.

http://forums.mozillazine.org/viewtopic.php?f=48&t=3037114

To paraphrase,

Go to Tools|Options|Advanced|General→Config Editor. Say "Yes" to the popup window telling you to be careful.

Enter layout.css.devPixelsPerPx in the search bar.

Double-click the result & increase/decrease the value until what you see on the screen is acceptable to you. This works for Thunderbird as well.

I'd suggest changing it to 1.0, then going up in steps 1.1, 1.2, 1.3 and so on. It gets rather bigger very quickly!

I think a drawback of this method is that message text will get bigger too.

Thanks. I'll try our IT department. I appreciate your quick response!