Really I do not understand the question. But like your other two it really in not containing a sufficient explanation of what you are doing for me to guess anything.

what i was trying to say was before i forward an email to someone i wanted to delete any other email addresses within the email but it ended up deleting the entire email

What you using for email ? Idk if firefox have his email, if yes, i want try it, but the Foundation Mozzila don't have too much money, so cant do a better thing, and gmail don't good, but i have good two email you can use : Tutanota : Zero-Encrypt (idk how say it, but it cool) and respect privacy, and can do password for your email, for, if someone be hacked, You'll say the code for the real guy, and he can open it, with Tutanota account ! Proton mail : Is the same, lol.

@Charaf this is the Thunderbird support forum. So Firefox has nothing to do with anything and suggesting alternative email clients is not in the slightest bit helpful to users trying to make Thunderbird work.

@Wolfman It is a pain, but the forwarded header as in a table, so you have to be careful of the way you select that information. Perhaps the easiest method is to click into the table and use the menu to select Format > table > delete>table and get rid of all the from to and the original subject. Otherwise you can select in the table and repeated the deletion process above, but choose delete the row selected instead of the whole table.

You can also select the text in cells ( bordered in blue when selected) and press the delete key. to clear out the text in those cells.

There is also the careful selection of only the text in the table cell (bordered in red when you first click forward), but you can not select the whole line like you would in a normal text situation.