PSA: On stock mobile Firefox, you can bypass Mozilla's lockout of about:config using the alternate URL:
chrome://geckoview/content/config.xhtml
Note that "chrome" here has nothing to do with the Google browser. Google stole the name from what was previously the Mozilla-internal name for "browser UI components".
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When you say stock do you exclude Nightly?, because I have no problem with accessing about:config.
@ekg Nightly doesn't have it blocked afaik.