Mozilla has issued 11 patches for its Firefox browser, nine of them being rated as critical.
The two remaining issues have been ranked as moderate and high.
A critical vulnerability "can be used to run attacker code and install software, requiring no user interaction beyond normal browsing," Mozilla said in an advisory note.
One of the critical patches fixed a vulnerability - CVE-2010-0179 - which had previously been addressed but still be exploited.
Another was described as covering "miscellaneous memory safety hazards."
A total of 68 bugs were also fixed in the browser to improve its functionality.
Firefox 3.6 - 3.6.13 is the most recent iteration - was this month ranked as the second most used browser by NetMarketShare.
It took 20.67 per cent of browser market share, behind Microsoft's Internet Explorer 8 on 34.63 per cent.
Mozilla has also issued a range of bug fixes for its Thunderbird email client.
"These releases fix several problems with large email folders stored on the user's computer as well as several fixes to improve performance, stability and security," the company said.
"Thunderbird 3.0.11 is the last security and stability update for Thunderbird 3.0.x."
The updates came ahead of a big Patch Tuesday for Microsoft, set to go ahead on 14 December.
A total of 17 fixes will be issued, most of which have been ranked as either critical or important.
