While XUL served its purpose in the early days of Mozilla, Firefox is looking to move away from XML User Interface Language, but doesn't yet know what will replace it.
Dave Camp, director of engineering for Firefox, said in a mailing list post that discussions on dumping XUL were preliminary, and many unanswered questions -- such as what to use instead, and the impact it would have on add-on developers -- need to be dealt with.
"XUL was our attempt to fill the gaps HTML had at for building large-scale web applications," Camp said. "Over time, the web -- and app development for the web -- has evolved its own set of standards and technologies; we should follow it."
Camp said that since XUL and XBL [XML Binding Language] were not web technologies, they fell behind HTML in priority, and it was hard for web developers to get up to speed.
"They don't get the same platform attention that HTML does (for good reason)," he said. "Performance problems go unfixed and it creates a lot of unnecessary complexity within Gecko.
"It's further from the web, and that doesn't help anybody."
In a post made minutes earlier, Camp said that Mozilla had undertaken a review of some Firefox features called "Great or Dead", which would involve improving a feature, removing it, or partnering with a third party that could implement it better.
"Every feature in the browser should be polished, functional, and a joy to use. Where we can't get it to that state, we shouldn't do it at all," he said.
"e10s is the only way to get the kind of snappy experience we need to make Firefox feel great," he said. "It needs work, not just in Firefox, but large parts of the Gecko as well. It also affects work outside of the core Mozilla code -- add-ons in particular are going to need a lot of work to adapt to the e10s world."
"We're close, but it's going to take some effort to get over the line."
Camp said that in the future, Firefox would focus on improving its private browsing mode and handling new features that result from partnerships in a better fashion.
"Folks said that Pocket should have been a bundled add-on that could have been more easily removed entirely from the browser," Camp said. "We tend to agree with that, and fixing that for Pocket and any future partner integrations is one concrete piece of engineering work we need to get done."
Over recent years, Mozilla has expanded its focus from Firefox to creating a mobile operating system targeting developing nations and low-performance handsets dubbed FirefoxOS, as well as attempting to monetise Firefox through advertisements and partnerships.