Extension like any software may reach end of life or end up abandoned. Currently I'm not finding on about:addons or elsewhere that extensions show a visual indication or notice to users when they are considered end of life by the developers or the developers haven't logged into the Developer hub https://addons.mozilla.org/en-US/firefox/extensions/ for some determined period of time. The period of time since last login could be one of a few indicators they have abandoned the extension. While yes some extension may work, it would help to ensure there is an active steward of the extension or enable indicating the extension may need a steward.
For instance the EFF's https everywhere extension is considered sunset as of January 2023. (per https://www.eff.org/https-everywhere/set-https-default-your-browser -> https://www.eff.org/deeplinks/2021/09/https-actually-everywhere suggesting just enable it in the browser by default). While it remains functional it is no longer supported and a replacement is available in the browser settings.
The goal here is to 1. keep users informed about the state of extensions support and compatibility monitoring 2. encourage a users to review extensions that may no longer be updated or tested for security, privacy, or performance 3. enable non-technical users know what maintenance users might need to take action to keep the browser performing as expected.
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It would be nice to highlight if an extension hasn't been updated in, say, a year? That seems reasonable.
Seconded on highlighting that the extension hasn't been updated.
There needs to be a field for developers to check "this extension will not be updated anymore". This doesnt mean it is outdated actually, but may be.
Also Firefox could do a compatibility check if the addons apis and whatever are fitting. No technical knowledge here, but Thunderbird has these indicators.