Middle-click or {Ctrl + click} a link to have it open in a new tab.

Or, right-click the link and select Open Link in a New Window (or New Tab).

Window size isn't going to change when opening a link, if you want it Maximized, that is a separate click if the original window isn't already Maximized.

Full Screen is window by window, and is toggled by F11.

Thanks for the reply and suggestions. It doesn't fully answer my question.

Tool/Options/ General Tab and Tools/Options/ Tab I can set all email to open in another window / tab but not maximised full screen as with F11

Please Advise Further: Is there an option hidden somewhere so all email always open displayed full screen ?

No, there isn't a feature in Firefox to have windows which are opened, to open in Full Screen mode.

I am in search for the same or at least similar answer to this question. Now, it's hard for me to believe there isn't a setting for this. Let me explain.

I use Firefox to access aol.com for my email. I have a 23" monitor. When I open an email in AOL, a new window appears in the top left corner of my screen that is approximately 6x6 inches in size. REGARDLESS of what the exact size is .... SOMETHING ... SOMEWHERE in the Firefox program is saying ... open an new window and make it "this size".

It has to be doing that. A computer can't (yet) come up with a window size based on its own thoughts. Somewhere that window size is being designated. So .... knowing where this setting is, is the real question. Changing it should be very easy once finding the setting is established. But, logic says there has to be a setting.

It is annoying to have to grab the sides of the window and drag them larger to read each email. Makes no sense that this can't be adjusted. Sorry.