I have my default font set to Helvetica. Quite often when I edit a message and type enter in the middle of it to insert a new paragraph Thunderbird changes it to variable-width. I understand it should not happen, since I am just inserting a paragraph in the middle of my text which is already correctly formatted. It also happens sometimes if I am writing and I go back a few paragraphs and insert a paragraph there too, not only when I multitask and click back on the compose window. I looked everywhere for a solution to make Thunderbird consistent in that aspect but I can't find anything except the basic "change your account settings, composition, html style, font" and everything is ok there and the application behaves normally if I don't make any mistake and I don't have to edit anything. The issue also happens when I am replying to messages and I also need to add paragraphs on my own text, which is already properly formatted using Helvetica, Thunderbird just ignores it and rolls it back to variable width.