I think if I use "shift plus enter" it creates a new line. Maybe that changed a while back and I didn't catch it?
Testing shift plus enter.
Got curious because I use Firefox as well and it does not have issues with new lines. In the icons above the post that let you set formatting and emojis, the last button looks like [ h ] and says 'Toggle view' when you mouse over it. When I have that on, it does weird things to new lines and strips out line breaks like you describe. I'm not sure if it's a toggle that you'd hit once and it turns off for all new posts, or, if it's the result of a checkbox in your profile settings.
Look under your account settings->Look and feel and see if the Show WYSIWYG by default is checked. If it is, uncheck it.