Please visit this link ... https://support.mozilla.com/en-US/questions/898304#answer-289192 Someone asked this question, I posted there the following question also......... 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. I will say this ... It's little wonder no one answered this. In fact it's little wonder anyone can find anything the way this forum software is configured. Wow, this is getting frustrating. I only hope I can find where I am now to see if there is an answer.