''TheOldFellow [[#question-1199463|said]]'' <blockquote> Up until a recent update (57.0.4) I could drag firefox windows between screens without issues. Now they automatically shrink to very tiny and useless. Chrome doesn't do that, not does Opera. I need to be able to switch this 'feature' OFF. Please stop changing the well established behaviors without warning!!!!! Linux Mint 18.3, NVidia GTX1080Ti. </blockquote> So Mozilla does not tell you what to do.....and why would you think that shrinking to "tiny" was a feature ? SAFE MODE *https://support.mozilla.org/kb/Safe+Mode *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes In Firefox Safe mode these changes are effective: *all extensions are disabled (about:addons) *default theme is used (no persona) *userChrome.css and userContent.css are ignored (chrome folder) *default toolbar layout is used (file: localstore-safe.rdf) *Javascript JIT compilers are disabled (prefs: javascript.options.*jit) *hardware acceleration is disabled (Options > Advanced > General) *plugins are not affected *preferences are not affected TEST.......if works fine then Extensions or Profile Please Refresh but do this 1st: Delete your Cookies and Cache and TEST. *https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles then *https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings TEST....... If no issues then Extensions which need to be added back in 1 at a time and tested ..... Or it is your Profile : Make a new one and test ...: *https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles If is your Profile : *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile *http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox Note: Any customization will revert back to default, you will also need to reinstall Extensions. Please let us know if this solved your issue or if need further assistance.

OK, I discovered that if I drag the firefox window to the new screen and pull it right to the top, then it retains the full-screen. So I have a solution. I didn't try Pkshadow's complicated diagnostics, but thanks for them anyway - useful for the next time.