I am using "Diamond" Stardock theme and after Firefox update buttons that close and minimize window disappear when browser is windowed:
Also some white edges appear on the sides.
I've excluded Firefox in WB settings but it only helped with fixing those buttons on fullscreen mode. Only changing the theme (some work, some don't) or unloading WB completely works on windowed mode.
Is there any way to fix this temporarily?
Under no circumstances must you exclude firefox from WB. Doing so will break things as you have seen.
Make sure you are not using any custom firefox themes as they could impact things. By the looks of it your firefox is not stock.
Generally speaking firefox is happiest with a theme with a titlebar that's not too tall.
Having checked here with diamond I see no issues with the titlebar buttons when maximized. They appear just fine, though when maximized the black inactive tab text is a pain, but again that appears to be a poor design decision by the firefox team.
Edit: One option is to enable the titlebar option in firefox by right clicking the titlebar and picking customize and clicking the Titlebar button at the bottom left.