simply changing jemalloc to mimalloc seems like mimalloc consumes less resources and is faster
**Quick Answer:** Firefox primarily uses **jemalloc** as its memory allocator because it improves performance, reduces fragmentation, and handles multithreaded workloads efficiently. While **mimalloc** (developed by Microsoft) is another modern allocator with strong performance, Firefox has not adopted it as its default.
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Firefox and Memory Allocators
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What is a Memory Allocator?
- A memory allocator manages how applications request and release memory at runtime.
- It affects **speed**, **fragmentation**, and **overall stability** of programs.
- Common allocators: **jemalloc**, **mimalloc**, **tcmalloc**, and system defaults like `malloc`.
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Firefox and jemalloc
- **jemalloc** was introduced in **Firefox 3** to replace the standard `malloc`.
- It was designed by **Jason Evans** for FreeBSD and later adopted by Firefox.
- **Key benefits for Firefox**:
- **Reduced fragmentation**: Keeps memory usage more compact, lowering RAM consumption.
- **Better multithreading support**: Handles concurrent allocations efficiently.
- **Performance improvements**: Faster allocation/deallocation compared to system `malloc`.
- **Predictable scaling**: Works well under heavy workloads like rendering complex web pages.
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Mimalloc (Microsoft)
- **mimalloc** is a newer allocator created by Microsoft Research.
- It is **compact, fast, and consistent** across benchmarks.
- Outperforms or matches jemalloc, tcmalloc, and others in many scenarios.
- However, Firefox has **not switched** to mimalloc as its default allocator.
- Mimalloc is more commonly used in Microsoft projects and experimental builds.
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Comparison Table
| Feature | jemalloc (Firefox default) | mimalloc (Microsoft) |
|---|
| **Adoption** | Widely used in Firefox, FreeBSD, Rust | Used in Microsoft projects, experimental adoption |
| **Performance** | Strong, optimized for multithreading | Very strong, often faster in benchmarks |
| **Fragmentation** | Low, stable under heavy loads | Very low, compact memory footprint |
| **Maturity** | Older, battle-tested | Newer, still gaining adoption |
| **Integration** | Deeply integrated into Firefox | Not default in Firefox |
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Trade-offs and Considerations
- **jemalloc** is deeply integrated into Firefox’s architecture, making it reliable and stable for millions of users.
- **mimalloc** shows promising results but would require extensive testing and integration work before replacing jemalloc in Firefox.
- Switching allocators can affect **performance consistency**, **debugging tools**, and **cross-platform compatibility**.
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In summary: **Firefox relies on jemalloc for its proven stability and performance. Mimalloc is a strong alternative allocator, but it’s not currently used as Firefox’s default.**
Would you like me to also explain **how jemalloc works internally** (its algorithms like arenas, bins, and slabs) so you can see why Firefox chose it?
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Firefox and Memory Allocators
What is a Memory Allocator?
Comparison Table
Trade-offs and Considerations
In summary: **Firefox relies on jemalloc for its proven stability and performance. Mimalloc is a strong alternative allocator, but it’s not currently used as Firefox’s default.** 
