Lock Firefox out of the Customizer due to file download heisenbug

[cvrebert]

Interim solution to #15016.
CC: @twbs/team for review

[XhmikosR]

I'm not in favor of this. The reason is that while many people seem to have this issue, others don't have it. Locking them out too is bad IMO.

Instead, we should warn only.

[mdo]

Disabling the customizer for everyone in Firefox sounds a bit harsh, but if y'all don't see a way around it, I'm not sure I do either.

[cvrebert]

@XhmikosR The bug seems to occur randomly; I've had the customizer succeed and fail at different times with the default variable values on the same computer. I guess it's a matter of how bad you consider "always fails" vs. "randomly fails"; a Firefox-specific warning is another option, yeah.

[XhmikosR]

I know it's random; I'm a Firefox only user myself. But it's too harsh to do this especially being that we can't really follow if/when this is fixed.

On the other hand, come v4, this shouldn't matter, right?

[cvrebert]

On the other hand, come v4, this shouldn't matter, right?

Right.

[hnrch02]

I think a warning would suffice in this case. Simply pointing out that the builds can randomly fail in Firefox and that a different browser would be the better choice to build Bootstrap.

[cvrebert]

Will open a PR to add a warning callout instead.