Yes, you can do that. Infact that's one the main benefits of QTP, it can be used to check your application against multiple browsers.
Regarding the pop-up, it has nothing to do with QTP. You need to uncheck "Remember passwords for my sites" in Tools--> Options--> Security in Firefox.
Regarding the pop-up, it has nothing to do with QTP. You need to uncheck "Remember passwords for my sites" in Tools--> Options--> Security in Firefox.
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