~750ms Extention Suspend by uBlock0 on `fetch` requests from other tabs while in MS Teams call

[user583953]

Prerequisites

• I verified that this is not a filter list issue. Report any issues with filter lists or broken website functionality in the uAssets issue tracker.
• This is NOT a YouTube, Facebook or Twitch report. These sites MUST be reported by clicking their respective links.
• I performed a cursory search of the issue tracker to avoid opening a duplicate issue.
• The issue is not present after disabling uBO in the browser.
• I checked the documentation to understand that the issue I am reporting is not normal behavior.

I tried to reproduce the issue when...

• uBO is the only extension.
• using a new, unmodified browser profile.

Description

fetch requests in any tab get delayed by ~750ms when an MS Teams call is active in another tab.

Notably, network monitor's timings show 0ms for such requests.

Firefox Profiler shows:

264ms Extension Suspend
Details: onBeforeRequest http://127.0.0.1:9999/ by uBlock0@raymondhill.net (chanId: 1192)
Track: Parent Process

251ms Extension Suspend
Details: onHeadersReceived http://127.0.0.1:9999/ by uBlock0@raymondhill.net (chanId: 1192)
Track: Parent Process

250ms Extension Suspend
Details: onHeadersReceived http://127.0.0.1:9999/ by uBlock0@raymondhill.net (chanId: 1192)
Track: Parent Process

idle Firefox 2026-05-11 13.42 profile.json.gz

Profiler screenshots (idle)

in_call Firefox 2026-05-11 13.45 profile.json.gz

Profiler + network monitor screenshots (in call)

A specific URL where the issue occurs.

https://example.com/
http://127.0.0.1:9999/

Steps to Reproduce

Reproducible with:

• fetch inside JS files loaded via <script src="..."> tags.
• fetch("https://jsonplaceholder.typicode.com/todos/1").then((r) => { document.body.innerHTML = "request finished" }) in F12 console.
• fetch("http://127.0.0.1:9999").then((r) => { document.body.innerHTML = "request finished" }) in F12 console.
• fetch to any other URL in F12 console from any page.

Optional means that the issue is reproducible both when this step is taken or not.

1. Create a new Firefox profile, install uBlock Origin (keep default settings).
2. (OPTIONAL) Disable uBlock Origin for both MS Teams and Tab B (via trusted sites or uBlock icon).
3. (OPTIONAL) Disable all filter lists in uBlock Origin (shows 0 network + 0 cosmetic).
4. Open Tab MS: https://teams.microsoft.com/v2/, then login.
5. Open Tab B: http://127.0.0.1:9999 or https://example.com/ or any other.
6. Tab B: Complete any of "reproducible with" (result: no delay, request completes immediately).
7. Tab MS: Join a call.
8. Tab B: Complete any of "reproducible with" (result: significant delay).

Expected behavior

I expected request to complete immediately regardless of being in call.

Actual behavior

When in call, requests are delayed by ~750 ms.

Configuration

Details

uBlock Origin: 1.70.0
Firefox: 150
filterset (summary):
 network: 169223
 cosmetic: 42871
 scriptlet: 33276
 html: 3231
listset (total-discarded, last-updated):
 default:
  user-filters: 0-0, never
  ublock-filters: 51968-69, 1h.43m
  ublock-badware: 11240-29, 1h.43m
  ublock-privacy: 4009-2, 1h.43m Δ
  ublock-unbreak: 2820-1, 1h.43m Δ
  ublock-quick-fixes: 468-13, 1h.43m Δ
  easylist: 95153-155, 1h.43m
  easyprivacy: 55834-33, 1h.40m Δ
  urlhaus-1: 25060-6, 1h.43m
  plowe-0: 3516-1028, 1h.43m
filterset (user): [empty]
userSettings: [none]
hiddenSettings: [none]
supportStats:
 allReadyAfter: 155 ms (selfie)
 maxAssetCacheWait: 48 ms
 cacheBackend: indexedDB

[gorhill]

Your profiling results show no WebExtensions process, where uBO resides, so I can't investigate anything related to uBO with these profiling results. Where exactly do you see "750ms Extention Suspend by uBlock0" in these results?

[user583953]

Your profiling results show no WebExtensions process, where uBO resides

Apologies, here's corrected export: in_call attempt 2 Firefox 2026-05-11 16.20 profile.json.gz

Where exactly do you see "750ms Extention Suspend by uBlock0" in these results?

In original profile: There are three ~250ms suspends in a row in Parent Process, visible in screenshot 3 under "Profiler + network monitor screenshots (in call)" details.

In attempt 2 profile: There are two ~250ms suspends in a row, this time first one is related to 127.0.0.1:9999 and the second one is for something MS Teams related. The end of MS Teams related suspend coincides with end of 127.0.0.1:9999 network request.

Attempt 2 profile screenshot

[gorhill]

I won't be able to reproduce your steps, I am not a user of MS Teams and I wouldn't be able to call anybody anyways.

In the latest profiling results, I don't see any uBO code running in the WebExtensions process, the only thing happening in there seems to be the process waiting for a call to __syscall_cancel_arch to complete:

uBO's own code shows nowhere in the WebExtensions process, so this means there is something else in the WebExtensions process preventing uBO's code to execute in a timely manner, and this explains why the parent process is waiting for uBO to process network requests.

The way forward is to find out what it is in the WebExtensions process which is hogging the CPU, preventing extensions code to execute. You can verify the issue also exist with other content blockers.

[user583953]

You can verify the issue also exist with other content blockers.

Reproduced with this tiny extension being the only one installed, the delay appears the moment I enable it:

background.js:

browser.webRequest.onBeforeRequest.addListener(() => {
    return {};
  },
  { urls: ["<all_urls>"] },
  ["blocking"]
);

manifest.json

{
  "manifest_version": 2,
  "name": "webrqtest",
  "version": "1.0",
  "description": "my desc",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "background": {
    "scripts": ["background.js"],
    "persistent": true
  }
}

I'll report that on Firefox's issue tracker. Thanks for helping.