Probably caused by T195973 (@Volker_E )

The action fields are using display:table-cell, which means they need a display:table parent. They did when first implemented, but that appears to have been lost during various refactors.

Esanders: Looks like it was removed in 24fe991c00. ...though I can't see any evidence that the .oo-ui-actionFieldLayout-field class that's removed there would ever actually have been created by ActionFieldLayout. So.

Change 443659 had a related patch set uploaded (by DLynch; owner: DLynch):
[oojs/ui@master] ActionFieldLayout: parent of input/button needs display:table

https://gerrit.wikimedia.org/r/443659

Change 443667 had a related patch set uploaded (by Bartosz Dziewoński; owner: Bartosz Dziewoński):
[mediawiki/extensions/VisualEditor@master] ve.ui.MWTemplatePlaceholderPage: Avoid Firefox rendering issue

https://gerrit.wikimedia.org/r/443667

Since only one weird case is affected (ActionFieldLayout with align: 'left' and no label), I would rather work around this in VE (it makes no difference to use align: 'top' instead; I submitted a patch), and figure out what caused this new issue before we make changes (I think nothing relevant changes in our code, we should try to bisect, or it might be a Firefox bug).

Change 443667 merged by jenkins-bot:
[mediawiki/extensions/VisualEditor@master] ve.ui.MWTemplatePlaceholderPage: Avoid Firefox rendering issue

https://gerrit.wikimedia.org/r/443667

Change 443659 abandoned by DLynch:
ActionFieldLayout: parent of input/button needs display:table

Reason:
Solved less generally in VE itself

https://gerrit.wikimedia.org/r/443659