[If this question was previously answered, I am unable to find it.] The Setup: I have dual displays: a 2021 iMac 27 inch (used as the start-up display of course) and a 2009 iMac 27inch. The Problem: Firefox windows change resolution when the window is moved between monitors. Windows from applications from Apple (Safari...! etc.) and other (e.g., Word, Excel, Powerpoint, Zoom, Adobe Elements) all retain their resolution when they are moved to the second display. Firefox does not, and windows are displayed at a significantly reduced resolution. I have been unable to alter any display settings or Firefox settings to eliminate the "blow-up" loss of resolution when the window is moved. (Yes, the text and display can be improved using the text/display commands in Firefox, but this kludge adjustment means the window displays poorly when returned to the first monitor.) I don't understand why Firefox resolution behaves this way when the windows in all the other applications I've listed it is unchanged, particularly Safari which is displaying HTML for the same graphics and text.