Thunderbird adds linebreaks by default at 72 characters per line.
These are not visible from Thunderbird, but someone looking at your emails from gmail mobile, will see the linebreaks. It looks messy and makes it painful to read.
To remove this, go to Edit>Preferences or Tools>Options> and at end of page “Config Editor”.
Search for variable “mailnews.wraplength” and set the value to “0”.
(As reference, default is 72)