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Firefox 60 ESR (the current basis of Tor Browser 8) officially doesn't support "legacy" browser extensions using XPCOM/XUL, only the newer WebExtension API. https://www.mozilla.org/en-US/firefox/60.0esr/releasenotes/#changed Tor Browser still includes some legacy extensions; apparently what makes them keep working is a extensions.legacy.exceptions pref (legacy/trac#26127 (moved); thanks sukhe for knowing that). I don't see where !meek-http-helper@bamsoftware.com is being allowed (edit: probably a [comment:4 source patch], thanks mcs), but somehow it is still working too.

Assess whether it's possible to rewrite the helper as a WebExtension, and do it if so. Ideally it will be possible to keep 100% compatibility with the current helper interface; but changing meek-client and meek-client-torbrowser is also an option.