Firefox? 2025 at the latest. Firefox was the project that deprecated ALSA a few years back. Now that all major distros ship with Wayland as the default, it's time to assume Wayland and not assume X. Eventually X support will become burdensome, and be removed.
The majority of users are still using x11 which is why for example Firefox just added hardware decoding under X last month. Optimistically for wayland this might change between 2024-26 at which point application developers can stop supporting it at which point people who are so inclined can provide an x11 build of Firefox in 2026-2028 which are liable to work through 2030.
In a less optimal world we are still in step one in 2030.
Anyone who thinks X is going anywhere this decade is dreaming.
When about do you think firefox will no longer build for X?
Python 3 was ready in 2008 but 2 will be supported by RHEL until at least 2024 16 years later. I will be surprised if in 2030 I can't simply pick distro with X and fire up firefox. Its kind of silly to expect users who are disinterested in using foo to pitch in to make foo acceptable when said users really want bar because you tell them that bar is now the standard "get over it".