Anthropic's Claude freezes only on Zen browser

[io-PEAK]

Preliminary Checks

• I have read and understood the important section above.
• I have searched existing issues and avoided creating duplicates.
• I am not filing an enhancement request.
• I have checked that this issue cannot be reproduced on Mozilla Firefox.
• I have checked that this issue can be reproduced once I removed all my Mods and Custom CSS.

What happened?

When using Claude.ai in Zen Browser, the tab becomes very slow and CPU usage spikes to 100%.

Zen shows the warning:
"This page is slowing down Zen. To speed up your browser, stop this page."

While this happens, Claude appears to still be generating the response in the background, but the UI becomes unresponsive.

If I wait about 1–2 minutes, press "Stop", and then refresh the page (Cmd + R), the full Claude response appears immediately. This suggests the response generation finished but the UI failed to update properly.

During this time my Mac becomes very hot due to the high CPU usage.

Expected behavior

Claude responses should render normally without freezing the tab or triggering the "page slowing down Zen" warning, and CPU usage should remain normal.

Actual behavior

The Claude.ai tab becomes extremely slow and CPU usage spikes to around 100%. Zen shows the "This page is slowing down Zen" warning and the response only appears after stopping and refreshing the page.

Steps to reproduce

1. Open Zen Browser
2. Navigate to https://claude.ai
3. Ask Claude a question that generates a long response
4. Wait while the response is generating
5. Zen shows the warning "This page is slowing down Zen"
6. Open Task Manager in browser
7. CPU usage spikes to 100% and the tab becomes slow
8. Wait 1–2 minutes
9. Click "Stop" and refresh the page (Cmd + R)
10. The full Claude response appears immediately

Screenshots and videos

Screen.Recording.2026-03-06.at.7.25.03.PM.mp4

Version

1.19.1b

What platform are you seeing the problem on?

macOS - aarch64

What component is this issue related to?

Performance

Relevant log output if applicable

[Propheticus]

Happens to Firefox users as well for answers that take longer to generate https://bugzilla.mozilla.org/show_bug.cgi?id=1993839

Have you already tried clearing the Claude site's data and cookies?

Some reports from webcompat:
Firefox: webcompat/web-bugs#210397
Librewolf: webcompat/web-bugs#209907
Floorp: webcompat/web-bugs#204747

[mr-cheffy]

I'll close this as its clearly an issue with gecko in general.

[io-PEAK]

Yes, I already tried clearing the site's data and cookies, so there is no solution for this?

as well for answers that take longer to generate

Not true, I tested by prompting just "Hi" in a new chat, spiked CPU usage to 100%.

[Propheticus]

Not true,

The report I link to does have multiple users state it's mainly noticeable on replies that take several seconds.
and uh

so there is no solution for this?

Not one that we can provide; a fix in either Firefox engine and/or Claude site.

[io-PEAK]

It seems random on my side. Sometimes it works for one or two prompts in the chat before the issue appears again. Thanks for the clarification.