Update page to current firefox' capabilities

[ceisserer]

for years multi-threading has been disabled running in firefox, despite the technical issues have been solved for 2,5 years. update the article to reflect the current situation. it seems even mozilla contacted google regarding the issue 8 months ago and nothing has changed. https://bugzilla.mozilla.org/show_bug.cgi?id=1633244

Fixes #SOME_ISSUE_NUMBER

Changes proposed in this pull request:

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[rachelandrew]

@tomayac could you take a look at this, and if correct make a CL on the migrated site?

[tomayac]

I have just removed all (potentially) outdated statements about other browsers. The document works as such.