User Agent: Mozilla/5.0 (Windows NT 6.1; rv:45.0) Gecko/20100101 Firefox/45.0 Build ID: 20160407164938 Steps to reproduce: Create and edit email message. Mouse around and do typical edits and typing. Editor might suddenly change behaviour without rhyme or reason. [ NL = New line, NP = New paragraph ] Actual results: Does not work consistently or predictably: - Sometimes pressing enter at end of current paragraph moves the entire current paragraph down (by one paragraph) instead of inserting new-paragraph at end of line. (Seems to occur after inserting something or backspacing.) - Sometimes pressing enter at end of current paragraph splits the entire current paragraph into two paragraphs. - Sometimes switches mode during the session such that pressing ENTER suddenly always inserts NL instead of NP. - When backspacing in a section (paragraph) of multiple lines (NL terminations), sometimes removing a NL works for the current line and inserts a NP at the next line. Removing that NP converts it to a NL and inserts NP at next line, repeatable for as long as the sequence of lines. - Typing text and pressing enter to add NP, then CTRL-Z to undo the NP, undoes the entire previous text too! - Pasting something at end of a line always inserts NL first (I first noticed this on previous version 38 too). Used to be able to just paste (append) onto end of the text. Expected results: << Expectations are sometimes included above in Actual >> << Some expectations are obvious by definition in Actual >> Enter at end of current paragraph... -- Should never split current paragraph or affect any preceding text. When backspacing in a section... -- I expect backspace to remove immediately preceding character only, without changing format of subsequent lines. Typing text and pressing enter to add NP, then CTRL-Z to undo... -- I would expect NP to signal start of a new undo block, such that CTRL-Z would undo only the NP. A second CTRL-Z would undo previous item(s), and so on.

Thanks for the report. We are tracking paragraph problems here: bug 1248971. Some of what you mentioned will be fixed in TB 45.1 scheduled for release next week. In general, please one bug report per problem.