[solved] Firefox 151 and webserial not working (Adafruit and EspHome)

red-lichtie · 2026-06-06 19:22:44

I wanted to test the new webserial functionality with esphome (https://web.esphome.io/) but it says:

"Dashboard Unavailable
ESPHome Web requires a browser that supports WebSerial. Please open this website on your desktop using Firefox, Google Chrome or Microsoft Edge."

Then I tried https://adafruit.github.io/Adafruit_WebSerial_ESPTool/ and I get:

"ESP Web Flasher loaded.
Error: can't access property "requestPort", serialLib is undefined"

Is there a configuration change I need to do to make it work (my account is in uucp)?

There is a policy configuration at /etc/firefox/policies/policies.json for "p11-kit-trust". Firefox default behaviour is to disable webserial if any policy is defined, so this file breaks it.
I added this to install my own CA certificates in firefox.

Last edited by red-lichtie (2026-06-07 08:27:24)

seth · 2026-06-07 06:10:18

Just a guess: noscript or adblocker extension interference?
Afair the press coverage you're supposed to get a dialog asking for permission for webserial.
Have you searched about:config for webserial ?

red-lichtie · 2026-06-07 07:12:52

Nice idea, next rabbit hole.

In about:config the option dom.webserial.enabled is locked to false, i.e. there is no option to change it.

This is on a PC at home and there are no enterprise policies enforcing DefaultSerialGuardSetting, so I'm expecting this to work out of the box. It would be weird to require a policy file to enable an option.

From what I've read Mozillas hacks and blog it should just work.

Is there maybe a serial feature that has to be enabled in packaging?

StarWolf3000 · 2026-06-07 07:27:28

Would still check about:policies, also does it work on a fresh profile?

red-lichtie · 2026-06-07 07:39:21

I didn't even know that was an option. There is only one policy there:SecurityDevices p11-kit-trust "/usr/lib/pkcs11/p11-kit-trust.so", I didn't set it so I'm guessing that that is a standard value.

I tested on a new profile with no plugns when it didn't work.

I also checked the official flatback, it is disabled there too but I don't know if that is some kind of flatpak standard.

red-lichtie · 2026-06-07 07:46:28

Looking at the admin page it states:

If any policies are set, the WebSerial API will be blocked by default. To allow usage of the API, set this policy to 3.

Is the Linux default p11-kit-trust triggering this?

I've never set a policy, so I have no idea where to go from here.

red-lichtie · 2026-06-07 08:00:46

I know I didn't create this:

pacman -Qo /etc/firefox/policies/policies.json
error: No package owns /etc/firefox/policies/policies.json

Contents:

cat /etc/firefox/policies/policies.json
{
  "policies": {
    "SecurityDevices": {
      "p11-kit-trust": "/usr/lib/pkcs11/p11-kit-trust.so"
    }
  }
}

Going to try updating this and restarting ff

red-lichtie · 2026-06-07 08:06:33

It works after updating it to:

{
  "policies": {
    "SecurityDevices": {
      "p11-kit-trust": "/usr/lib/pkcs11/p11-kit-trust.so"
    }
    ,
    "DefaultSerialGuardSetting": 3
  }
}

So now the question, who/what created that file?

StarWolf3000 · 2026-06-07 08:17:21

Maybe it was you, you need it to add certificate authorities to Firefox: https://support.mozilla.org/en-US/kb/se … es-firefox

red-lichtie · 2026-06-07 08:30:44

StarWolf3000 wrote:

Maybe it was you, you need it to add certificate authorities to Firefox: https://support.mozilla.org/en-US/kb/se … es-firefox

On the nose, it was me, and exactly for that reason. Original post updated to reflect this.

It seems strange though that the default is to disable it if any policy, even an unrelated one, is enabled.

Last edited by red-lichtie (2026-06-07 08:31:08)

Arch Linux

#1 2026-06-06 19:22:44

[solved] Firefox 151 and webserial not working (Adafruit and EspHome)

#2 2026-06-07 06:10:18

Re: [solved] Firefox 151 and webserial not working (Adafruit and EspHome)

#3 2026-06-07 07:12:52

Re: [solved] Firefox 151 and webserial not working (Adafruit and EspHome)

#4 2026-06-07 07:27:28

Re: [solved] Firefox 151 and webserial not working (Adafruit and EspHome)

#5 2026-06-07 07:39:21

Re: [solved] Firefox 151 and webserial not working (Adafruit and EspHome)

#6 2026-06-07 07:46:28

Re: [solved] Firefox 151 and webserial not working (Adafruit and EspHome)

#7 2026-06-07 08:00:46

Re: [solved] Firefox 151 and webserial not working (Adafruit and EspHome)

#8 2026-06-07 08:06:33

Re: [solved] Firefox 151 and webserial not working (Adafruit and EspHome)

#9 2026-06-07 08:17:21

Re: [solved] Firefox 151 and webserial not working (Adafruit and EspHome)

#10 2026-06-07 08:30:44

Re: [solved] Firefox 151 and webserial not working (Adafruit and EspHome)

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