WebAssembly in Firefox: What Is and Isn't Supported (2024-2025 Status)

Firefox has had excellent WebAssembly support since Wasm became standardized. Here's what Firefox offers for Wasm development and where it stands compared to Chrome.

Firefox's WebAssembly Implementation

Firefox uses SpiderMonkey's Wasm engine, which has supported the core Wasm spec since 2017. Key characteristics:

• Baseline compiler (Cranelift): Fast compilation for quick startup
• Optimizing compiler (Ion): Profile-guided optimization for long-running Wasm
• Tiered compilation: Start with baseline, upgrade to optimized in background

The two-tier approach means Wasm code starts fast and gets faster as it runs — good for both short-lived and long-running workloads.

What Firefox Supports (2024-2025)

Firefox leads or ties with Chrome on most Wasm proposals:

Core Features (Full Support)

• MVP: Core Wasm (functions, memory, tables)
• Wasm threads: SharedArrayBuffer + Atomics (requires COOP/COEP headers)
• SIMD: Fixed-width SIMD (128-bit)
• Bulk memory operations: Fast mem.copy/mem.fill
• Reference types: Externref, funcref
• Tail calls: Proper tail call optimization

Extensions (Firefox Support)

• GC proposal: Wasm garbage collection — Firefox 120+ has shipping support
• Relaxed SIMD: Extended SIMD instructions — Firefox ships
• Exception handling: Try/catch in Wasm — Firefox ships
• Multi-memory: Multiple memory instances — Firefox ships in Nightly
• Memory64: 64-bit Wasm memory addresses — Firefox ships in Nightly

Firefox-Specific

• dump() function: Available in Wasm modules during debugging (non-standard, useful in Firefox context)
• SpiderMonkey JSPI: JS Promise Integration proposal — in development

Firefox DevTools for WebAssembly

Debugging Wasm

Firefox's Debugger has native Wasm support:

1. Open DevTools → Debugger
2. Wasm sources appear in the Sources pane (with source maps if provided, or as WAT disassembly)
3. Set breakpoints in WAT code
4. Step through execution
5. Inspect local variables and memory

The debugger shows Wasm local variables (i32, f64, etc.) in the Scopes pane. You can inspect individual memory pages in the Memory tab.

Memory Profiling for Wasm

Wasm linear memory shows up in the Memory tab. To inspect:

1. DevTools → Memory → Take Snapshot
2. In the snapshot, Wasm memory is labeled "WebAssembly.Memory"
3. You can view raw bytes, though interpretation requires knowing your layout

Using about:memory

about:memory shows Wasm-specific memory breakdown:

• explicit/js-non-window/runtime/wasm/code/... — compiled Wasm code size
• explicit/js-non-window/runtime/wasm/data/... — runtime data
• explicit/js-non-window/runtime/wasm/tables/... — function tables

Useful for diagnosing whether Wasm modules are being cached/reused or recompiled.

Wasm Threads: The Header Requirement

Wasm threads require SharedArrayBuffer, which requires Cross-Origin Isolation:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

Without these headers, SharedArrayBuffer is undefined and threaded Wasm won't work. This was introduced after Spectre. Both Firefox and Chrome require this.

Test if your page has COOP/COEP enabled:

console.log(crossOriginIsolated); // true if headers are set correctly

Emscripten and Firefox

Most C/C++ → Wasm workflows use Emscripten. Firefox compatibility:

• Standard Emscripten output runs fine
• The -s ENVIRONMENT=web flag generates code compatible with all browsers
• POSIX threading in Emscripten requires the COOP/COEP headers mentioned above

Emscripten's -s ASSERTIONS=2 mode works in Firefox — it adds runtime checks that show helpful error messages in the browser console.

WASI and Firefox

WASI (WebAssembly System Interface) is for running Wasm outside browsers, but browser-side WASI is an active area:

• @bytecodealliance/jco can run WASI components in browsers via JS adapter layer
• Firefox doesn't have native WASI support (no browser does yet)
• The WASI proposal for browser environments is in early stages

Performance Tips for Firefox

1. Use optimizing compiler paths: Wasm running for >1ms gets the optimizing compiler. Short-running Wasm may not benefit from optimization.

2. Avoid integer/float type conversions at the boundary: JS ↔ Wasm boundary has overhead. Batch data transfers using typed arrays rather than many small calls.

3. SIMD for data-parallel work: If you're doing image processing, audio processing, or simulation, use Wasm SIMD. Firefox's SIMD support is mature.

4. Memory alignment: Access aligned memory (4-byte aligned for i32, 8-byte for i64/f64). Unaligned access is valid in Wasm but slower.

5. Avoid growing memory during hot paths: memory.grow is slow. Pre-allocate if possible.

Firefox Wasm vs Chrome Wasm

Startup speed: V8's Liftoff (Chrome's baseline compiler) is faster for initial compilation. Firefox's Cranelift is competitive.

Peak performance: Both engines produce excellent machine code. Firefox's Ion JIT and Chrome's Turboshaft/TurboFan are roughly equivalent for most Wasm workloads.

Standards coverage: Firefox often ships experimental proposals earlier. For cutting-edge Wasm (WasmGC, Relaxed SIMD), Firefox may lead.

Debugging: Firefox DevTools has strong Wasm debugging. Chrome DevTools is also good. Both are mature.

Bottom line: For Wasm development, test on both. Firefox is a strong Wasm platform, and ensuring your Wasm runs on Firefox is important for web standards compliance.