how to customize notification message

Are you talking about the message for an out-dated browser version or within the plugin’s backend?

I see. There’s a possibility to add custom notifications (cf. http://browser-update.org/customize.html, options: „text“ and „text_xx“) – since then I refused implementing it in order not to confuse less tech-savvy users. Will think about an implementation…