[Arnavion]

If the GPL code is an integrated part of your code, then you've created a derivative work, a "work based on the Program" as the GPL calls it. In this case your work must also be licensed as GPL.

>5. Conveying Modified Source Versions.

>You may convey a work based on the Program, or the modifications to produce it from the Program, in the form of source code under the terms of section 4, provided that you also meet all of these conditions:

>[...]

>c) You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged. This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it.

It seems to be the case here since, as the top comment by RraaLL says, they've included GPL-licensed JavaScript from uBO in their extension.