The migration (from 27) itself was rather smooth , PM 28 started upgrading one addon (I do not care for cookies 2.8.9 > 2.9.1) and accepted all the rest; and the videos previously no longer working now work .
Then I did a careful check of the Preferences (moved from Edit to Tools), running one instance on the old system in "setenv DISPLAY" alongside an instance on the new system, noted the differences and consulted the spot help (would you believe I never noticed before that each Preferences screen/tab has an online help) ... In general I found that most new preferences have an acceptable default, and changed only a couple or so.
Finally I had a look at the about:config, which is inherited from layers of previous installations back to the Palaeolithic. Maybe some even to Firefox. I wonder whether some of those entries are obsolete and could be removed. I report only the first tokens in the property name"
- extensions.firebug... (firebug is no longer installed since ages)
- distribution.opensuse... (suspect inherited from age old firefox)