User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:50.0) Gecko/20100101 Firefox/50.0 Build ID: 20161208153507 Steps to reproduce: Compose email Actual results: Occasionally while composing, the font changes, in size and style. This happens spontaneoulsy, without any input from me. Sometimes, after remaining in the new font for a while, it will revert to the font I was using before the the font changed. I'm not sure I understand the Version menu. I am using v 45.5.1 (x86) Expected results: The font should have remained as I had set it.
I don't think the font is changing "spontaneously". You must be clicking somewhere and you pick up the font at the cursor. See for example bug 1319907, which I duped to bug 1270803. Worth reading: Bug 1270803 comment #1.
Reopened, since the reporter wrote in bug 1270803 comment 8: === Font size and style have changed while I am typing, without clicking anything. I have been just happily typing along, and abruptly the size AND style of the font have changed simultaneously. And then, after a while, it may spontaneously return to the original font. If this is controled by Mozilla core software, I wonder if my program has been corrupted. If so, might removing and reloading the Thunderbird program fix it? Although this doesn't happen frequently, just occasionaly. Could a corruption produce this problem inconsistently? === I've never seen such an effect. Does this happen regularly or predictably? Does it happen when you run Thunderbird without add-ons, see Help menu to start with add-ons disabled.
The spontaneous change in font is unpredictable. I may go a few weeks without seeing it, and then it happens again. Running Thunderbird without add-ons didn't occur to me. At the age of 73, I have lost some of my marbles. However I am largely dependent on Classic theme restorer; the new GUI is optimised for small screens, but less functional on large screens. I am not familiar with the new UI.