Greasy Fork

In the #vidprogress container, there is nothing visible...
What's should i do to make it active ?

I joint the settings for your script.
Waterfox Classic / greasemonkey

Please keep in mind that, as described, this script is for YouTube's new layout only. i.e. Polymer. It won't work for old YouTube layout.

Also, it's recommended to use either ViolentMonkey or TamperMonkey browser addon.

Yep, i have the YouTube's new layout.
The problem was Greasemonkey and as you said - after testing - with Tampermonkey /Waterfox Classic, it work fine!
Thanks...
Possible support for Greasemonkey?

No, sorry. Greasemonkey is known to restrict the capabilities of UserScripts for the sake of "security" since version 4.0, and it has no option to remove the restriction - making it incompatible even with scripts which were created for older Greasemonkey versions. I can't fix that since I have no control over other people's code.

ViolentMonkey browser extension used to be restrictive as Greasemonkey, but later it provides an option to be less restrictive. This script support that option (i.e. ViolentMonkey's custom @inject-into page metadata). Greasemonkey doesn't have anything like that.

Thanks for the explanation.

I don'y like the v.4, that's why i continue to use Greasemonkey v.3.16...
I am a "Classic" retrograde user.
:-)

Anyway, for your script, using Tampermonkey too is good.