Hello,
Regarding the sync - are you talking about the thumbnails synchronization? Because the dials are synchronized as a whole, not by groups. Please contact me to my support e-mail.
I can advice you also about the CSS if you have a specific question. If you know CSS, you can just write it in the Options page to the "Custom CSS styles:" field.

Best regards,
Juraj Mäsiar

Hello,

I'm not sure I understand, but if you want to be able to scroll the thumbnails, you can go to Options page, and to the field "Custom CSS styles:" enter this:

.dials > a > div {
overflow-y: scroll;
margin-right: -29px;
}

That will enable scrolling of the dials, plus hides the scrollbar.
Alternatively, if you want to see scrollbar and only when it's really needed (when the image is long enough), you can use this:

.dials > a > div {
overflow-y: auto;
}

If you mean something else, please contact me to my support e-mail.

Best regards,
Juraj Mäsiar

Hello,

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.

Best regards,
Juraj Mäsiar

Hello,
Firefox 66 is now migrating add-ons data to a new storage and there can be cases when you loose you data:
https://www.ghacks.net/2019/02/16/firefox-extensions-to-use-different-storage-type-in-firefox-66/

If this is the case, go to dials page / click the upper right corner menu and select "History / backups" - these backups are stored in a different storage and should not be affected by the migration.

If this doesn't help, please contact me to my support e-mail.

Also note that downgrading Firefox will also clear your addons data (https://www.ghacks.net/2018/08/06/dont-downgrade-firefox-63/)

Best regards,
Juraj Mäsiar