Extension is good,
Is usefull only need 1 click extension to go next page.
I keep press extension, then I can move to back page and next page by using alt + left / alt + right.

Read 1 comic chapter that have 10 page /comic/1 /comic/2 /comic/3
I click extension 10 times then use alt left / right so I dont need click extension anymore.

Very good idea!!
Your extension is very useful to watch or download galleries of pictures.
I have a feature request to enhance your extension:
The extension is currently incrementing the number located at the extreme end of the url, if existing.
But often, the number is not at the end of the URL, for instance :
www.domain.com/test01.html => www.domain.com/test02.html etc...
www.domain.com/test01initial/ => www.domain.com/test02initial/ etc...

Could it manage incrementation of the first number detected starting from the right of the url?

This extension is quite useful as is. But could you make it more like the Chrome extension URL Plus?