I have also posted this question in the Think Different Stack Exchange Q&A site https://apple.stackexchange.com/questions/485754/workaround-for-1password-7-password-autofill-breaking-on-firefox-147-0-1 , and sent a message to the 1Password support team.

1Password dropped support for 1Password 4.x over 3 years ago. It relied on a non-standard feature in Firefox that was deprecated years before, but because of the unsupported status, the 1Password authors did not publish a version with a fix.

Firefox 147 inadvertently disabled the deprecated functionality, but it is possible to re-enable it. Warning: although re-enabling the functionality would restore the functionality, it will not address any other security issues that may exist in the 1Password 4.x extensions.

For details on the issue and the work-around, see https://bugzilla.mozilla.org/show_bug.cgi?id=2013772#c15

Thank you for this reply, Rob Wu! You appear to be very well informed. I appreciate your taking the time to answer this question.

Rob writes, "Firefox 147 inadvertently disabled the deprecated functionality, but it is possible to re-enable it. Warning: although re-enabling the functionality would restore the functionality, it will not address any other security issues that may exist in the 1Password 4.x extensions."

I am away from the computer with Firefox for a few days, so I can't test this yet. However, it looks promising. The linked bug report, and the vulnerability disclosure linked from there, is interesting reading.

A detail: the version of 1Password I am using is 7.x, not 4.x. However, it might be that what really matters is the version of the Firefox extension which 1Password uses. 1Password limits their current extension to 1Password 8 and newer, so those of us who want to stick with a previous 1Password version are forced to resort to the older, unsupported extension. That extension may present itself as 1Password 4.x.

[edited to fix a typo and add missing words. 2026-02-09]

I have now had a chance to read the bug report #2013772 to which Rob Wu linked. Here is what I understand.

Firefox 147 did have a change in behaviour. It was intentional, though the rollout of the change appears to have stumbled a little. It is possible, for now, to reverse the change. In the long term, the change will likely be harder to evade.

The way to reverse that change in behaviour is to:

1. In a browser tab, visit `about:config` . A "Proceed with Caution" warning appears.
2. Click the "Accept the Risk and Continue" button. An empty preferences page, with the warning, "Changing these preferences can impact Firefox performance or security.", appears.
3. In the text box at the top of the page, enter `dom.keyboardevent.init_key_event.enabled_in_addons` . A row with the same name appears below the text box. In the centre of the row is a word, either `true` or `false`. At the right is a double-arrow icon.
4. Firefox 147 initially has the word `false` in this row.
5. Click the double-arrow icon. The word `false` changes to `true`.
6. Close the browser tab. Firefox immediately has the pre-147 behaviour.

As Rob Wu says, "For details on the issue and the work-around, see https://bugzilla.mozilla.org/show_bug.cgi?id=2013772#c15".

The bug discussion brings up two important points.

First, a crucial piece of this behaviour is a Firefox extension named `onepassword4@agilebits.com.xpi` (122,878 bytes, Version 4.7.5.90, Last Updated May 5, 2022). This lets the 1Password application communicate with the Firefox browser. The current version of this extension, published on addons.mozilla.org as "1Password: Password Manager" by the makers of 1Password, works only with version 8 of the 1Password application. I am using 1Password version 7, which uses the older extension, which is confusingly named `onepassword4`. The distinction between the Firefox extension and the 1Password app is important.

Second, this 2022-vintage extension is written for older versions of Firefox. Current Firefox has stopped supporting some technical interfaces which the extension uses. This is reasonable, but it is a problem for those of us using the old extension.

The Firefox team's position is that we should use the most recent version of the 1Password extension. The 1Password team's position is that we should move to the most recent version of the 1Password app in order to use the most recent extension. But this version of 1Password takes away a capability which I value.

Someone had a clever idea: modify the old extension, without the help of the 1Password team, so that it works with the current versions of Firefox. The bug ticket has a link to a modified extension published on addons.mozilla.org. I love the idea, but I don't love the way the people behind the modified extension published it. They did not provide enough transparency to confidently recommend it to others.

For now, I have configured my copy of Firefox 147.0.1 to set `dom.keyboardevent.init_key_event.enabled_in_addons` to `true`. That is good enough for me.