Well the previous password manager allowed to get a list of all your saved passwords, and even sort them alphabetically.
Now it only shows passwords by domain and requires you to type the master password to display each one, every single time.
This was helpful to keep track of duplicated passwords reused on more than one site, either to fix them or update them. Now they have made password management difficult.
Another thing they removed is the ability to set when to expire history. People have different preferences, some would like their history to expire after 1 day, others may like to keep history for 30000 days, now it is internally decided by Mozilla when will your history expires.
Taking away choices is not a good practice.
And like you commented, there is some kind of phobia now to put things in dialog boxes, it has to either be on a bar or a tab.
Seamonkey was a browser/suite used by a small group of people (less than 0.2% of the market share?), that chose to use Seamonkey specifically for how it worked, and how it looked. Making this random changes will make people to continue using older vulnerable versions or finally change to a popular browser.