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TL;DR:
Open a new window, and 25 seconds later, this window will start functioning again. Drag all your misbehaving tabs over to this new window, closing the old windows.

Firefox still has the tendency to occasionally freeze and require a restart, but I'm not sure that this is actually related or made more likely, by the typing/input issue. So "fixing" the input lag by creating a new window does still seem to offer a functional work-around.

TL;DR:
Start a new window by dragging a tab away from your misbehaving window. Roughly 25 seconds later, this new window will start functioning again. Drag all your misbehaving tabs over to this new window, and they will immediately start working again too.

Firefox still has the tendency to occasionally freeze and require a restart, but I'm not sure that this is actually related, or made more likely, by the typing/input issue. So "fixing" the input lag by creating a new window does still seem to offer a functional work-around.

Edit3: It doesn't seem the issue is caused by any of the plugins, as it also appeared with the other half of my plugins deactivated.

Firefox still has the tendency to occasionally freeze and require a restart, but I'm not sure that this is actually related or made more likely, by the typing/input issue. So "fixing" the input lag by creating a new window does still seem to offer a functional work-around.

17:37:27.561 Error: Connection closed, pending request to server0.conn0.watcher2.process4//styleSheetsActor476, type getText failed Request stack: request@resource://devtools/shared/protocol/Front.js:299:14 generateRequestMethods/</frontProto[name]@resource://devtools/shared/protocol/Front/FrontClassWithSpec.js:47:19 _fetchSourceText@resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:287:40 baseFrontClassDestroy resource://devtools/shared/protocol/Front.js:105 destroy resource://devtools/shared/protocol/Front.js:73 destroy resource://devtools/shared/protocol/Pool.js:213 baseFrontClassDestroy resource://devtools/shared/protocol/Front.js:109 purgeRequests resource://devtools/client/devtools-client.js:679 onPacket resource://devtools/client/devtools-client.js:470 send resource://devtools/shared/transport/local-transport.js:73 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103 StyleSheetEditor.sys.mjs:342:17   Request stack: request@resource://devtools/shared/protocol/Front.js:299:14 generateRequestMethods/</frontProto[name]@resource://devtools/shared/protocol/Front/FrontClassWithSpec.js:47:19 _fetchSourceText@resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:287:40 baseFrontClassDestroy resource://devtools/shared/protocol/Front.js:105 destroy resource://devtools/shared/protocol/Front.js:73 destroy resource://devtools/shared/protocol/Pool.js:213 baseFrontClassDestroy resource://devtools/shared/protocol/Front.js:109 purgeRequests resource://devtools/client/devtools-client.js:679 onPacket resource://devtools/client/devtools-client.js:470 send resource://devtools/shared/transport/local-transport.js:73 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103 StyleEditorUI.sys.mjs:1574:15   Error: Target already destroyed, unable to fetch children fronts getFront resource://devtools/client/fronts/targets/target-mixin.js:235 _getStyleSheetsFront resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:946 _fetchSourceText resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:279 fetchSource resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:333 #addStyleSheetEditor resource://devtools/client/styleeditor/StyleEditorUI.sys.mjs:712 promise resource://devtools/client/styleeditor/StyleEditorUI.sys.mjs:576 StyleSheetEditor.sys.mjs:342:17 

Edit to add:

I've just disabled some of these, as a test, they were all active when the issue appeared before.

17:37:27.561 Error: Connection closed, pending request to server0.conn0.watcher2.process4//styleSheetsActor476, type getText failed Request stack: request@resource://devtools/shared/protocol/Front.js:299:14 generateRequestMethods/</frontProto[name]@resource://devtools/shared/protocol/Front/FrontClassWithSpec.js:47:19 _fetchSourceText@resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:287:40 baseFrontClassDestroy resource://devtools/shared/protocol/Front.js:105 destroy resource://devtools/shared/protocol/Front.js:73 destroy resource://devtools/shared/protocol/Pool.js:213 baseFrontClassDestroy resource://devtools/shared/protocol/Front.js:109 purgeRequests resource://devtools/client/devtools-client.js:679 onPacket resource://devtools/client/devtools-client.js:470 send resource://devtools/shared/transport/local-transport.js:73 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103 StyleSheetEditor.sys.mjs:342:17  

Request stack: request@resource://devtools/shared/protocol/Front.js:299:14 generateRequestMethods/</frontProto[name]@resource://devtools/shared/protocol/Front/FrontClassWithSpec.js:47:19 _fetchSourceText@resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:287:40 baseFrontClassDestroy resource://devtools/shared/protocol/Front.js:105 destroy resource://devtools/shared/protocol/Front.js:73 destroy resource://devtools/shared/protocol/Pool.js:213 baseFrontClassDestroy resource://devtools/shared/protocol/Front.js:109 purgeRequests resource://devtools/client/devtools-client.js:679 onPacket resource://devtools/client/devtools-client.js:470 send resource://devtools/shared/transport/local-transport.js:73 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103 StyleEditorUI.sys.mjs:1574:15  

Error: Target already destroyed, unable to fetch children fronts getFront resource://devtools/client/fronts/targets/target-mixin.js:235 _getStyleSheetsFront resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:946 _fetchSourceText resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:279 fetchSource resource://devtools/client/styleeditor/StyleSheetEditor.sys.mjs:333 #addStyleSheetEditor resource://devtools/client/styleeditor/StyleEditorUI.sys.mjs:712 promise resource://devtools/client/styleeditor/StyleEditorUI.sys.mjs:576 StyleSheetEditor.sys.mjs:342:17 

Edit to add:

I've just disabled some of these, as a test, they were all active when the issue appeared before.

Edit2: The issue just reappeared, so it seems that it wasn't Bitwarden, Floccus or PassFF... I've now Disabled Privacy Badger and uBlock, lets see if that prevents it from reappearing.

Also, it seems like the "fix" of moving into a new window, doesn't fix it very long term. Both times I've used this I still had to restart a few minutes later. One time Firefox just fully hung, another time the delay reappeared.