Gecko and Firefox have been using Bugzilla for more than 25 years at this point. There's a lot of internal workflows, tooling and processes that are really dependent on the specific functionality in Bugzilla. I think it would be an extremely high risk project to try and replace Bugzilla with GitHub issues.
That said, there are also other teams and projects who do use GitHub for issue tracking. However the closer to Firefox/Gecko you are the harder this gets. For example it's hard to cross-reference GitHub issues with Bugzilla issues, or vice versa. I've seen people try to build two-way sync between GitHub and Bugzilla, but there are quite considerable technical challenges in trying to make that kind of cross-system replication work well.
However your point that GitHub makes issue submission easier for people who aren't deeply embedded in the project is a good one. I'm directly involved with webcompat.com, which aims to collect reports of broken sites from end users. It's using a GitHub issue tracker as the backend; allowing developers to directly report through GitHub, and a web-form frontend so that people without even a GitHub account can still submit reports (as you can imagine quite some effort is required here to ensure that it's not overwhelmed by spam). So finding ways to enable users to report issues is something we care about.
However, even in the webcompat.com case where collecting issues from people outside the project is the most important concern, we've taken to moving confirmed reports into bugzilla, so that they can be cross-referenced with the corresponding platform bugs, more easily used as inputs to prioritization, etc. That single source of truth for all bugs turns out to be very useful for process reasons as well as technical ones.
So — (again) without being any kind of decision maker here — I think it's very unlikely that Firefox will move entirely to GitHub issues in the foreseeable future; it's just too challenging given the history and requirements. Having some kind of one-way sync from GitHub to Bugzilla seems like a more tractable approach from an engineering point of view, but even there it's likely that there are non-trivial costs and tradeoffs involved.
That said, there are also other teams and projects who do use GitHub for issue tracking. However the closer to Firefox/Gecko you are the harder this gets. For example it's hard to cross-reference GitHub issues with Bugzilla issues, or vice versa. I've seen people try to build two-way sync between GitHub and Bugzilla, but there are quite considerable technical challenges in trying to make that kind of cross-system replication work well.
However your point that GitHub makes issue submission easier for people who aren't deeply embedded in the project is a good one. I'm directly involved with webcompat.com, which aims to collect reports of broken sites from end users. It's using a GitHub issue tracker as the backend; allowing developers to directly report through GitHub, and a web-form frontend so that people without even a GitHub account can still submit reports (as you can imagine quite some effort is required here to ensure that it's not overwhelmed by spam). So finding ways to enable users to report issues is something we care about.
However, even in the webcompat.com case where collecting issues from people outside the project is the most important concern, we've taken to moving confirmed reports into bugzilla, so that they can be cross-referenced with the corresponding platform bugs, more easily used as inputs to prioritization, etc. That single source of truth for all bugs turns out to be very useful for process reasons as well as technical ones.
So — (again) without being any kind of decision maker here — I think it's very unlikely that Firefox will move entirely to GitHub issues in the foreseeable future; it's just too challenging given the history and requirements. Having some kind of one-way sync from GitHub to Bugzilla seems like a more tractable approach from an engineering point of view, but even there it's likely that there are non-trivial costs and tradeoffs involved.