Let me skip all the discussion why and head to what you may want to do next.
I use the (latest 15.0.1) Thunderbird email software.
Recently, a strange problem has started appearing while I am composing an email:
Whenever I copy/paste some words from an earlier typed portion of an email being composed, the pasted words appear in a LARGER font size so that I have to go through the annoyance of reducing back each time the font size of the pasted words.
I am not able to figure out the source of the above recent phenomenon and therefore a solution to the problem.
Thanks for the help.