This is most probably known Firefox Sync. issue - if you setup GroupSpeedDial as homepage on several devices - with Firefox sync. enabled - Firefox will synchronize wrong URL (every add-on has different URL on different device). I reported this bug a long time ago and it should be fixed in current Firefox (make sure you use at least Firefox 60 on all your devices): https://bugzilla.mozilla.org/show_bug.cgi?id=1412083
What should help is excluding Homepage from synchronization: https://support.mozilla.org/en-US/questions/919674 (basically just go to "about:config" page and change setting "services.sync.prefs.sync.browser.startup.homepage" to false)
If this doens't help, please contact me to my support e-mail.