# Will develop edition support legacy addon in the future?

**URL:** <https://discourse.mozilla.org/t/will-develop-edition-support-legacy-addon-in-the-future/23015>\
**Category:** Add-ons\
**Created:** [December 4, 2017, 8:05pm UTC](https://discourse.mozilla.org/t/will-develop-edition-support-legacy-addon-in-the-future/23015 "2017-12-04T20:05:49Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![yakineko](https://sea1.discourse-cdn.com/flex001/user_avatar/discourse.mozilla.org/yakineko/32/30781_2.png) [@yakineko](https://discourse.mozilla.org/u/yakineko)\
**Post date:** [December 4, 2017, 8:05pm UTC](https://discourse.mozilla.org/t/will-develop-edition-support-legacy-addon-in-the-future/23015/1 "2017-12-04T20:05:49Z")

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In [Mozilla Add-ons Blog](https://blog.mozilla.org/addons/2017/10/03/legacy-add-on-support-on-firefox-esr/), it says "The following ESR release (59), and any subsequent release, will not support legacy add-ons. There will be no override provided for this behavior."  
Does it means that legacy add-ons will not be supported in future develop edition, neither?  
If so, will there be any way other than webextensions, to modify(as legacy add-ons did) the UI of Firefox?(even if only in develop edition)

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**Author:** ![erosman](https://sea1.discourse-cdn.com/flex001/user_avatar/discourse.mozilla.org/erosman/32/15447_2.png) [@erosman](https://discourse.mozilla.org/u/erosman)\
**Post date:** [December 5, 2017, 6:46pm UTC](https://discourse.mozilla.org/t/will-develop-edition-support-legacy-addon-in-the-future/23015/2 "2017-12-05T18:46:39Z")

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> [@yakineko](#):
>
> Does it means that legacy add-ons will not be supported in future develop edition, neither?

Yes…

> [@yakineko](#):
>
> If so, will there be any way other than webextensions, to modify(as legacy add-ons did) the UI of Firefox?(even if only in develop edition)

Only via WebExtension APIs and they have a limited access to Firefox UI in comparison to legacy APIs.

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**Author:** ![jorgev](https://avatars.discourse-cdn.com/v4/letter/j/71c47a/32.png) [@jorgev](https://discourse.mozilla.org/u/jorgev)\
**Post date:** [December 5, 2017, 9:13pm UTC](https://discourse.mozilla.org/t/will-develop-edition-support-legacy-addon-in-the-future/23015/3 "2017-12-05T21:13:23Z")

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It’s currently possible to modify the Firefox UI using `userContent.css`. I’m not sure if that feature will continue to be supported in the long term. Additionally, there are big changes happening under the hood in Firefox that will make it nearly impossible to create legacy add-ons that modify its UI, even if they were forced enabled.

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**Author:** ![yakineko](https://sea1.discourse-cdn.com/flex001/user_avatar/discourse.mozilla.org/yakineko/32/30781_2.png) [@yakineko](https://discourse.mozilla.org/u/yakineko)\
**Post date:** [December 6, 2017, 6:55am UTC](https://discourse.mozilla.org/t/will-develop-edition-support-legacy-addon-in-the-future/23015/4 "2017-12-06T06:55:51Z")

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Thank you for the replies.  
So it looks pretty frustrating for me, since I was trying to develop an addon, which needs to modify the tab bar.  
I will try userContent.css first, but I don’t know if it does what I want. (I want to do something like TreeStyleTab, but not in a sidebar)  
I have seen that there will be an API for creating a toolbar, I hope it won’t be too troublesome to make a fake tab bar.