When some particular sites are visited, multiple proxy authentication prompts are received. After canceling these pop-up windows the page can be finalized visited, we have observed that this issue is happening when Adobe Flash player is involved.
This has been an intermittent issue over the last months, however there is a site that can be used to reproduce the problem.
You will NEVER completely fix this as the firefox uber nanny will not allow to disable these annying pop ups. Whay makes them think we want annoying pop ups? People have been trying to thwart pop ups since the invention of the internet.
Anyway you can try this addon: https://addons.mozilla.org/en-us/firefox/addon/skip-cert-error/
Its sort of like nags away, it will hopefully click the dialog for you so you dont have to be as annoyed with it,
Meet your Uber-nanny : http://www.mozilla.org/about/manifesto.en.html
From that page: Individuals' security on the Internet is fundamental and cannot be treated as optional. (so we will nag the crap out of you)
Individuals must have the ability to shape their own experiences on the Internet. (UNLESS we want to nag the cap out of you. For you own "security" ofcourse.
Thanks for your response. I should have mentioned two things, I am already using a recent firefox version (16.0.1) and these authentication prompts are for proxy user and password, these are not related with digital certificates as suggested in the first answer provided.
The problem that we are experiencing it is pretty similar to a well-known behaviour experienced in previous firefox versions. This can be replicated as follow:
0) Start firefox .
1) Open several sites in different tabs.
2) Close firefox selecting "Save and quit"
3) Start firefox again. Multiple authentication prompts (user and password) will be required.
It seems to me (speculating) that adobe flash plugin may be sending a burst of hits through the proxy, as none of them has been previously authenticated, one username/password request is being sent to the browser one for each "hit" that was simultaneously sent through the proxy.