Firefox randomly pegs one CPU core (70–100%) with "Isolated Web Content" even on simple websites

[No-9990]

I've been trying to get to the bottom of a Firefox performance issue, and after weeks of troubleshooting I'm running out of ideas.

The main problem is that Firefox will randomly drive one CPU core to 70–100% usage. When this happens, my laptop temperature quickly rises to around 80–90°C and the fans ramp up. The process responsible is usually "Isolated Web Content."

What's confusing is that this doesn't only happen on complex websites. I've seen it while watching YouTube videos, but I've also seen it on pages that appear to contain little more than basic HTML. At times it feels completely random.

What I've already checked:

No Firefox extensions installed.
Followed Mozilla's troubleshooting recommendations (safe mode, hardware acceleration tests, refresh profile, etc.).
The problem still occurs.
Other applications, including some 2D and 3D games, don't consistently drive my CPU this hard.

I'm not claiming Firefox is doing anything malicious, but I genuinely don't understand why a web browser can sometimes consume an entire CPU core for extended periods on relatively simple workloads. Is this expected behavior? Is it a known issue?

I've also noticed many posts where people report high CPU usage from "Isolated Web Content," but I rarely see a clear explanation of why it happens or a reliable fix.

Has anyone else experienced this in general?

In the meantime i will not use firefox... its all so tiresome...


Last note: instead of continuously adding new features (especially AI-related ones that increase complexity and background workload), it would be far more valuable to focus engineering effort on core performance and security. 

[No-9990]

Microsoft Teams in Firefox is often an extreme example of how heavy modern web applications have become. In practice it can push CPU usage very high and cause sustained load that even leads to serious thermal spikes (in my case close to 90°C), which is obviously unacceptable for something that is “just a web app”. It highlights the same underlying issue: instead of continuously adding new features and complexity, there should be a stronger focus on efficiency, resource control, and realistic performance budgets for web applications. Otherwise the browser ends up carrying the cost of increasingly bloated web platforms that behave more like full desktop software than lightweight pages.

As a side note, Chromium-based browsers seem to handle some of these workloads more smoothly in practice, which makes the contrast even more noticeable and raises questions about performance priorities and implementation choices across different browser engines.

[No-9990]

After even more testing, I've made my decision: I'm done with Firefox.

I tried several Firefox forks that disable many of Mozilla's built-in services, and CPU usage dropped by around 17% in my tests. That's a pretty significant difference. On top of that, Chromium-based browsers simply handle CPU-intensive websites much better on my machine. They don't constantly spike a core to 100% or make my laptop sound like it's about to take off.

I also found countless posts about causing high CPU usage, with reports going all the way back to 2016. Yet here we are in 2026 and people are still reporting the same issue. Maybe the root causes are different in each case, but as a user all I see is the same symptom over and over again with no reliable fix.

To be honest, this Connect platform doesn't exactly inspire confidence either. The view counter seems to increase every time you refresh the page, even from the same IP. That makes thread popularity look inflated and doesn't seem very meaningful. Then there are multiple different Mozilla community sites Connect, Discourse, Bugzilla, Support forums, Matrix, Reddit... where are users actually supposed to report things? Everything feels fragmented.

I really tried to give Firefox a fair chance. I spent weeks troubleshooting, tested without extensions, with a fresh profile, Safe Mode, hardware acceleration on and off, and followed all the recommended steps. Nothing consistently fixed the problem.

At this point I've had enough. I'm switching to another browser and I don't see myself coming back unless performance becomes a much higher priority. Features are great, but they don't matter much if the browser randomly pegs a CPU core, heats up the laptop, and drains the battery while browsing relatively normal websites.