This problem presented itself out of the blue. The Save As dialog box expanded to full screen, and although the arrows that appear when the mouse cursor is over the edge of a window are present, they flicker and do not grab the edge to pull it in. Every other window on the computer does not have an issue like this. Restarting Firefox or the computer does not eliminate the problem. I am hoping that there is a settings file somewhere that the numeric value for the dialog box size can be changed, since I do not want to completely reinstall the browser. As a side note, if the browser has been running many downloads for a while, it will usually start failing to display information, and eventually will get to a point where the Save As box flickers for an instant and disappears and the entire browser becomes unresponsive for an hour or more. Downloads continue (including the one that it froze up on, albeit as a randomly named file in User...Temp), but it is not possible to restart the browser until the downloads complete (which in some cases can be several hours). Usually I revert to using IE11 until I can restart Firefox. Perhaps one of my strongest dislikes of Firefox though is the issue with its resistance to copy and paste operations in the Save As dialog box. Most websites offer files with cryptic and even identical names for different files, so renaming files is a critical operation if one is to keep track of and later identify the files one has. A problem unique to Firefox, attempts to highlight and paste in a better file name usually are met with a refusal to paste, a stuck context menu that can take a minute of clicking to clear, repeated jumping to the end of the file name, and other similar issues. I have encountered this problem repeatedly on different computers and different operating systems, it is a problem I have only ever encountered in Firefox.