With Skype ‘number highlighting’ turned ON, a number on a website looks like this.
Turned OFF, it looks like this:

On Firefox, you can turn it on or off here:

You can disable the Skype features that allow these things (by default) in the first place. Activate Skype then go to Tools > Options > Advanced, then remove the check marks to all of the options here:

Why?
If you are developing websites you need to view your projects the way you intend them to look like. That is, without any ‘enhancements’ or aids that add anything to the website.
As a web developer I have encountered many times when a client would suggest removing or editing these ‘improvements’ (as Skype might call them), not knowing these were caused by Skype and not added by myself.