Firefox 158 Is Out: Faster Form Filling, Better PDFs and a Long List of Security Fixes
Mozilla released Firefox 158 on 13 October 2026. This is a release I think most Firefox users should install promptly, and then enjoy at leisure. The new features are useful in small, everyday ways. The security list is long. Mozilla published the full Firefox 158 release notes and a separate security advisory for Firefox 158, and I worked through both for this post.
Form filling that looks at your open tabs
The feature Mozilla leads with lives in Smart Window. When you click into a form field, or pick Fill from sources, Firefox can now suggest data based on your relevant open tabs and your saved autofill information. Think of booking a ticket in one tab while your details sit in another, or copying an address from an email into a delivery form. If Firefox can pull the right detail forward safely, that saves real minutes across a week.
Mozilla is rolling this out gradually, so your copy of Firefox may not show it on day one. That gradual rollout is sensible for a feature that touches personal data. When it reaches you, try it on a low stakes form first, read every suggested field before you submit, and check what it filled in. Autofill mistakes are usually funny. On a tax, visa or exam form, they are not. Keep your saved addresses and payment details tidy in Settings, because any autofill system is only as accurate as the data it holds.
PDFs get easier to read and share
Firefox now creates more accessible PDFs when you save a page as a PDF, and when you print on Linux and macOS. Mozilla says this also fixes a set of PDF rendering problems, including missing math symbols. Accessible PDFs matter more than most people realise. A properly structured PDF works with screen readers, searches cleanly, and keeps headings in order when someone copies text out of it. If you send price lists, study notes or invoices as PDFs, your readers get a better file without any extra effort from you.
Firefox 158 also adds support for the JPEG XL image format, which Mozilla notes generally compresses better than WebP, JPEG, PNG and GIF. That puts Firefox alongside Chrome, which added JPEG XL decoding in Chrome 155 earlier this month, so Firefox is catching up here rather than getting there first. Most readers will never choose an image format by hand. The practical effect arrives slowly, as more sites serve JPEG XL and pages get a little lighter. The New Tab page now keeps a library of your custom wallpapers under Your images, instead of only the most recent one, and Firefox translations add Albanian, Belarusian and Filipino. Scrolling no longer rounds to whole screen pixels, so movement should feel smoother and more precise on high resolution displays and at high zoom levels.
The security list is the real story
Mozilla rates the overall impact of the Firefox 158 security advisory as high. The advisory lists dozens of fixed flaws, including use after free bugs, sandbox escapes, JavaScript engine problems and privilege escalation issues across components such as Networking, Graphics, DOM and IndexedDB. Mozilla’s advisory lists 77 fixed flaws, CVE-2026-109704 to CVE-2026-109781 with one number in that range unused, which matches the 77 that PCWorld counted in its coverage of the release. A browser is the app most of us trust with banking, email, work documents and family photos, so a list like this is the strongest reason to update quickly rather than someday.
One detail caught my eye. Several of the JavaScript engine and graphics bugs were reported by Amy Burnett of OpenAI, according to Mozilla’s advisory credits. Browser makers now get serious vulnerability reports from AI labs as well as from independent researchers. That sits next to a wider trend I follow on this site, where European agencies are testing frontier AI models for cyber evaluation work. More eyes, human and otherwise, finding browser bugs before criminals do is good news for the rest of us.
Mozilla also fixed a favicon crash, improved menu readability under Windows High Contrast themes, and fixed a Network Monitor issue that could miss WebSocket messages arriving right after a connection opens, which developers will appreciate. One unresolved note: web apps launched with the third party PWAsForFirefox tool may fail to start and show an incomplete installation message. Mozilla says updating that tool to version 2.20.1 or later fixes it.
If you run Firefox on Linux, my Ubuntu 26.10 release rundown covers what changed on the desktop this month.
What to do
Update Firefox now. On desktop, open the menu, go to Help, then About Firefox, and let it check. On Android, take the update from Google Play. If you manage Firefox for a family member who tends to ignore update prompts, do this one for them and turn on automatic updates while you are there.
After updating, keep an eye out for the Smart Window form suggestions as the rollout reaches you, and try the improved Save as PDF on a page you would normally share. Small features, real fixes, and a safer browser. That is a good release in my book.
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