Update MANUAL_INSTALL.md with integration test note

[simonpoole]

Description

Fixes the most glaring omissions (some of them are mentioned in the contributing section, but that naturally doesn't really help with a step by step installation guide) for an Ubuntu 26.04 install.

Note: this doesn't fix the Fedora and MacOS instructions which likely have similar issues in a headless configuration.

[tomhughes]

Well the right thing to do would be to include firefox in the various install commands rather than just adding a comment?

As to the second part, was there something extra you had to do beyond just installing firefox to get WebGL support?

[simonpoole]

It isn't quite clear to me what assumptions the current authors of the text are making, I suspect that they are assuming a desktop Linux box and then none of this is likely an issue.

As to the second part, was there something extra you had to do beyond just installing firefox to get WebGL support?

For now I just ignored the issue (3 or 4 tests fail), but I'll investigate in a sec.

[simonpoole]

There's one bit that I need to add before this is good to go.

[simonpoole]

It isn't quite clear to me what assumptions the current authors of the text are making, I suspect that they are assuming a desktop Linux box and then none of this is likely an issue.

As to the second part, was there something extra you had to do beyond just installing firefox to get WebGL support?

For now I just ignored the issue (3 or 4 tests fail), but I'll investigate in a sec.

I've added some text on running x on a virtual framebuffer, interesting enough using the snap installed FF works better than the ESR version that the Docker image utilises.