I think it’s Firefox that changed rather than Opus, although the Firefox change is perfectly valid and I’m not blaming them.
I have a hunch that some of the programs changed how they open folders to improve compatibility with WINE. WINE doesn’t support the API they were using to open folders (probably because Microsoft have not documented what it actually does, which is quite arcane and varies between Windows versions), so maybe to help people running their software under WINE they are checking if the API succeeded and falling back on an alternative if it didn’t. Problem is, on Windows with Opus that was causing a delay and then a second window to open.
The fix we’ve been testing looks good so far and will probably be in Opus 10.0.3.1 (i.e. the first beta after the imminent 10.0.3.0 release), unless our testing shows up any problems.