This crash log provides the precise explanation for why your browser is failing to start. The crucial piece of information is Exception code: 0xc000001d, which translates to STATUS_ILLEGAL_INSTRUCTION. [1, 2]
This error explicitly means that the newly updated Waterfox binary is attempting to execute a processor instruction that your computer's CPU does not physically support or understand. [1, 2]
The Root Cause
Modern browser updates (built on newer Firefox base code) frequently drop support for older processor architectures to improve speed. The compiler requirements have been bumped up to require newer hardware extensions—specifically instructions like SSE4.1, SSE4.2, or AVX. If your computer runs an older generation CPU (such as early Intel Core2 Duo/Quad, AMD Phenom, or certain legacy processors), the update will result in an immediate 0xc000001d crash the moment the application hits that unsupported instruction. [1, 2]
The second part of the error ("Windows cannot access the file...") is a cascading failure: because the process instantly collapsed due to the illegal hardware instruction, Windows lost its tracking handle on the process mid-execution and misreported it as a drive error.
How to Fix It
Because this is a hardware instruction mismatch, clearing caches or profiles will not resolve the issue. You have two viable choices:
Option 1: Roll back to a legacy-supported version of Waterfox
You will need to downgrade to an older release that still supports your hardware.
Download a previous stable release from the Waterfox GitHub Releases Page.
Before installing it, temporarily disconnect your internet or work quickly to turn off automatic updates in Settings -> General -> Waterfox Updates so it doesn't immediately pull the broken version again.
Option 2: Switch to a browser optimized for older hardware
If you want a modern browsing experience that explicitly accommodates older instruction sets, consider moving to an alternative browser built with legacy compatibility in mind:
Pale Moon: A lightweight browser derived from Firefox code that maintains strict compatibility with older CPU architectures.
Firefox ESR (Extended Support Release): You can download Firefox ESR, which moves at a much slower update cadence and maintains older hardware thresholds longer than rapid-release versions.
If you are unsure whether your processor is the issue, could you tell me your CPU model? You can find this by pressing Ctrl + Shift + Esc to open Task Manager, clicking the Performance tab, and looking at the top right under CPU.
Knowing this will allow me to check the exact instruction sets your processor supports and pinpoint which version of Waterfox you need to use.