When/how is this happening? Does it happen on websites? some, many?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Hi Rex-Berwick, are you experiencing unwanted breaks between lines on a forum you access using Firefox, or is this question about composing a new message in Thunderbird?

Firefox and VBulletin

Older VBulletin sites have an issue with a change in Firefox 60 and later. To revert the change:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste newline and pause while the list is filtered

(3) Double-click the editor.use_div_for_default_newlines preference to switch the value from true to false (was false in Firefox 57-59)

Thunderbird

In the Options/Preferences dialog, under Compose, you can change Thunderbird from using HTML paragraphs to use body text. More details if relevant.