Did you try to only type the MP once and close the other dialog boxes via Cancel?

(sorry about the delay; was traveling)

Yes - I've tried canceling every dialog box but the first one - that'll leave the other tabs without access to the MP list (apparently). Also: since the tabs take a while to each load, the MP dialog box comes up in a staggered fashion - I need to wait for all 5 to show, or I end up typing parts of my 18-character password in all 5 dialogs (3 characters in the first, 5 in the second, etc.).

Apparently I'm the only one being bugged by this - trying to find a place to submit a feature request (feature enhancement): one MP dialog box no matter how many simultaneous tabs get opened - once one MP dialog is open, any other tab that wants to trigger the MP should just wait for it to be opened by the already-open dialog.

Not really. It is a known problem (I have it myself) and still present in current Firefox versions, but should hopefully be solved in the upcoming Firefox 4 version (possibly next February or March).

Bug 499233 - multiple master password prompts triggered by filling form logins in multiple tabs

(please do not comment in bug reports)

You could try this extension - StartupMaster
https://addons.mozilla.org/en-US/firefox/addon/9808/

@the-edmeister - I will check out the plugin - sounds just like the workaround I've been looking for.

@Cor-el - I think you're right RE FF 4.xb: I've been 50/50 between production FF and FF 4.xb - and the beta isn't giving me the same grief; hadn't noticed until now - although it occurs to me that FF 4.xb probably isn't asking me for a master password. Weird how it was able to get into my MP list; maybe it asked me once and then kept the password.

Did you create a new Profile specifically for Firefox 4.0beta?