Telemtry still seems to show a relative high rate of access to this property: https://mzl.la/32GgRiQ. Probably some kind of browser detection. Should we try removing it Nightly?

Yeah, putting this behind a preference and disabling it on Nightly/early Beta sounds good.

It's not safe to remove window.sidebar at all. Yeah, this is used for browser detection, as always on Browserhacks, which explains its high internet usage. It is clear that if we remove that, a lot of sites will break.

(In reply to janestrapper from comment #4)

It's not safe to remove window.sidebar at all. Yeah, this is used for browser detection, as always on Browserhacks, which explains its high internet usage. It is clear that if we remove that, a lot of sites will break.

Citation needed. Browserhacks mentions a lot of hacks that are not up-to-date anymore (like appearance or moz-tree-row-based hacks). Looking at telemetry it seems like usage is ~0, and we've gotten no issues reported in 11 months. Kagami, do you think we should proceed?

Hmm, I lie, it's more like ~0.5% (link) which is non-trivial. Then again if usages are like the ones in bug 1768345 (causing compat issues rather than fixing them) maybe it's worth a try. It's been disabled for 11 months on Nightly / early-beta after all.

Agreed, I believe many of existing browser detection cases are now invalid. I'll write a patch to disable it.

See Bug 1769458.

We will be monitoring for site breakage here, as there are signs that it might be bad to both keep or remove sidebar, from a webcompat perspective. But for now, we'll unset the webcompat priority flag on this bug until we have more info.