Thank you for your help.
I'm sorry, I was not very precise while describing the problem.
My tests were on Firefox running on windows 7 64 bits, Google Chrome running on Windows 7 64 bits too, and I also tested it on an iPad air 3 (ios8).
Here is an image showing the differences between the 3 browsers :
The first, chrome, is ok for the three tests.
The second, Firefox, fails in tweening the opacity with 'stroke-opacity' (third test).
The third, safari ios 8, fails in moving the dots in the first test.
Other tests :
Chrome on ipad is the same as safari on ipad (first fails).
Safari on macos is the same as safari on ipad (first fails).
Chrome on Macos is the same as chrome on windows (all good).
Firefox on macos is the same as firefox on windows (third fails).
I guess there is a bug with firefox and ios, but as you managed to reduce the differences between firefox and such with transform origin, I wondered if it could be the same for opacity
Thanks again anyway !
Hope it helps.