Mozilla explains Firefox crashes on Intel 13th, 14th Gen Raptor Lake systems [Update]

Earlier today Mozilla officially confirmed Firefox's new redesign for 2026 called "Nova" as it shared details about it. While it's not out yet, you can try it out on the newer Nightly updates.

Speaking of updates, the firm also released the latest stable (Release channel) version with 151.01 fixing a crash affecting some Intel Raptor Lake systems, mainly desktops. The issue has been under investigation for roughly a year since the first crashes were observed in Firefox Nightly builds of the time.

Mozilla engineers traced them down to failures in a zlib-rs compression routine where a 16-bit dist value appeared incorrect which consequently led to index out-of-bounds crashes. However, the team suspected the underlying cause involved Intel Raptor Lake CPU bugs specifically the RPL050 and RPL060 issues related to incorrect load behavior. Mozilla noted however that Intel’s newer 0x12c microcode appeared to reduce the crashes greatly.

For anyone wondering, according to Intel's own documentation RPL050 describes a scenario where one logical CPU core may fail to observe the latest data written by another core, while RPL060 involves incorrect data being returned from certain split-load memory operations spanning cache lines.

As a refresher, Raptor Lake had already been under fire from the press as well as other companies due to its instability issues. Mozilla's Gabriel Svelto had warned previously about issues like overheating that had the potential to make things far worse. He said: "If you have an Intel Raptor Lake system and you're in the northern hemisphere, chances are that your machine is crashing more often because of the summer heat. I know because I can literally see which EU countries have been affected by heat waves by looking at the locales of Firefox crash reports coming from Raptor Lake systems. ... Raptor Lake systems have known timing/voltage issues that get worse with temperature. Things are so bad at this time that we had to disable a bot that was filing crash reports automatically because it was almost only finding crashes from people with affected systems."

If that was the condition in Europe then the situation was likely far worse at places around the Indian subcontinent, which is closer to the Equator and hence much warmer. Regardless it is good to see that the fix is finally available for all users. The changelog for Firefox 151.01 is given below:

• Fixed a crash experienced by users with Intel Raptor Lake CPUs.
• Fixed an issue on Windows where some websites using WebSerial to flash device firmware could fail unexpectedly.

To download the update head over to the Neowin software stories page or the official Firefox website.

Update, May 26 2025, 15.15 GMT: It appears that Mozilla still has the issue "1950764" as open even though it is marked as a "fixed" issue as spotted by a Mastodon user Hugo.

As such Gabriel Svelto has clarified that the Raptor Lake CPU errors RPL050 and RPL060 were not the root cause of the issue, even though it was initially suspected. Investigation is still going on.