Re: when a tab opens in Firefox

I don't know any way to do this without polling:  that it, without having a timer or event that regularly checks to see if caption of active window matches *Google Keep - Mozilla Firefox*. 

 I'm assuming that firefox uses the active tab as its window caption like Edge and Chrome -- you should check.
 

 If so, set up a timer or event to regularly check caption of active window and if so do your command:  The timer reset/event command would be something like (not tested)
 if (win.matches(win.handle("active"),"*Google Keep - Mozilla Firefox*")
 // your command here
 

 

 The more often the timer/event checks, the better response you will get but also the more resources the checking will do.  So you will need to do a tradeoff.  I'd start with a timer once per second.