Only Mozilla's Firefox capped storage at 5MB and was not vulnerable, he found.
In the UK, research firm Nielsen suggests that IE still commands 70% of the market, with Mozilla's Firefox on 18%.
Some advised users to switch to alternative browsers, such as Google's Chrome or Mozilla's Firefox while the bug was being fixed.
The objection comes despite Internet Explorer's rapid loss of a sizeable portion of the web-browser market to Mozilla's Firefox and Apple's Safari.
Mozilla's Firefox browser has rolled out new features, such as the ability to integrate with social-networking sites so users can see updates and messages from Facebook Inc.
When the latest version of Mozilla's Firefox was released in June, bugs that allowed a site to install malicious programs on a visitor's computer were reported to security vendor Tipping Point within hours.
"There's no Firefox, no Internet Explorer - and there's not likely to be because of the development environment, " he added.
BBC: Opera iPhone app downloaded a million times in 24 hours
Mozilla's fledgling Firefox OS has yet to seed out to actual consumers, but when it does an "easy and secure" payment platform should be in place.
But instead of using Google's Gears, Jolicloud relies on Mozilla's Prism and Firefox to run various web-based applications.
ECONOMIST: Google goes in search of an instant operating system
Google's engineers contribute code to Firefox and pay the foundation a share of advertising when people search Google in the browser's toolbar.
As long as Google keeps the money flowing, Firefox's small team of developers moves quickly.
Like Firefox's, Chrome's source code is free for anybody to change and improve, and even for rival browser-makers to incorporate.
Firefox's new features include automatic updates (users previously had to complete an installation for each update) and extra pop-up ad blocking.
Firefox's market share jumped by a third in the last year to 19%, while Internet Explorer lost 6 percentage points to 73%.
Other big applications built on Firefox's underlying code include Songbird, an open-source version of Apple's iTunes, and Joost, a high-quality Web video channel.
Like other open source software, such as the operating system Linux, Firefox's code is freely available for any programmer to examine and improve.
After a brief stretch in beta followed by some vague teasing, Firefox's native Android app update is finally set to hit Google Play.
ENGADGET: Native Firefox Android browser adds speed, Flash, HTML5 and a fresh look (hands-on)
Whether Microsoft co-opts Firefox's best ideas, or the Web browser continues gobbling up market share, we'll all be using Mozilla's best ideas to access those new applications soon.
It's not unthinkable that Firefox OS could use a similar technique on its "apps" but we've heard nothing about it or the creation of the necessary infrastructure from Mozilla.
ENGADGET: Firefox OS is repeating the mistakes of others and hoping for a different outcome
Firefox's mobile operating system showed up on a mystery phone tonight at a pre-CES event ahead of its unveiling later this year, carrying no branding and looking light on features.
ENGADGET: Firefox OS shows up on a mystery phone, we go hands-on (update: now with video!)
Firefox's relationship with Google may see some stress over the next year or so as the search giant debuts its own browser, which will likely also be open to tinkering by amateurs.
Still, Firefox's advances have already had their intended effect: to present web surfers with a viable alternative to the monopoly, and thus to restart much-need innovation in browser technology, even by Microsoft itself.
The program still identifies itself (in brackets) as IE 11, but it forgoes Microsoft's older identifier ("MSIE") and simply describes itself as being a browser that renders HTML in a similar way to Firefox's Gecko layout engine.
Let's start with the basic premise of Firefox OS. Like Chrome OS, there's little more here than a web browser running on top of Linux.
ENGADGET: Firefox OS is repeating the mistakes of others and hoping for a different outcome
Feature-wise, preferences and other desktop settings imported easily with Firefox Sync's shared password system, and the unfortunately named "Awesome Screen" is the new home page shown above, from which it's fairly simple to launch your preferred sites.
ENGADGET: Native Firefox Android browser adds speed, Flash, HTML5 and a fresh look (hands-on)
At the annual CanSecWest security conference in Vancouver last March, the year-old Chrome was the only browser left standing after stalwarts like Internet Explorer, Firefox and Apple's Safari had been hacked to pieces Safari in literally seconds by security experts.
ECONOMIST: Google goes in search of an instant operating system
IE's share dropped to 92 percent when Firefox was launched but this was now back to 97 percent, Weeks said.
"It's still very much the case that Firefox is way ahead of IE when it comes to standard compliance, " he said.
The event, dubbed Mozilla Translathon 2012, was organized to provide translations for Firefox, Google's Endangered Languages Project, the WikiMedia software that powers Wikipedia and 500 crowdsourced articles, to boot.
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