Thanks for feedback. Summing up in my own words -
1) xwp1.0.9 with current FF and sliding focus works
2) xwp1.0.10 (all versions) do not
I'm pretty sure 1.0.10 (beta1) did have the same code which disables sliding focus for Mozilla/FF as beta4 but will check again.
All 1.0.10 versions (starting with beta1 - xwp-unofficial-1-0-10.exe) do have a lot of changes compared to 1.0.9 in different places. So it seems the difference is somewhere else than the particular 'sliding focus' code I thought.
We should remember these findings until AN releases the new 1.0.11 and check again.