Was support for the SeaMonkey browser just removed?

Question

For years I've been browsing the site using the SeaMonkey Gecko-engine-based browser. It has the following user agent:

Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Firefox/68.0 SeaMonkey/2.53.10

Suddenly this afternoon, interacting with most of the site of the site seems broken:

• I cannot expand vote counts.
• I cannot expand comment lists.
• I cannot add a comment (clicking "Add a comment" scrolls to the top of the page.)
• Previews when posting or editing a post do not appear.
• Clicking "Review your question" does nothing.

Things render OK, but I can't interact with them.

The change happened shortly after 2021-12-08 19:26:39Z when I was able to post an answer as usual.

I seem to be able to post from another browser with the following user agent:

Mozilla/5.0 (Windows NT 10.0; rv:91.0) Gecko/20100101 Firefox/91.0

Did Stack Exchange kill support for this browser today?

Update

I am able to edit a post I made recently, but when I do the preview area does not show, and I see one error and many warnings in the console window, beginning with the following:

Timestamp: 12/8/2021, 4:43:56 PM
Error: SyntaxError: expected expression, got '.'
Source File: https://cdn.sstatic.net/Js/full.en.js?v=c6f3a9a7390a
Line: 1, Column: 235329
Source Code:
Height>n.clientHeight?(n.classList.add("v-truncate-fade"),t?.classList.remove("v-hidden")):(n.classList.remove("v-trunca

Timestamp: 12/8/2021, 4:43:56 PM
Warning: Property contained reference to invalid variable.  Error in parsing value for ‘font-family’.  Falling back to ‘inherit’.
Source File: https://cdn.sstatic.net/Sites/stackoverflow/primary.css?v=ee6784011c19
Line: 1
Source Code:
var(--theme-body-font-family)

Timestamp: 12/8/2021, 4:43:56 PM
Warning: Error in parsing value for ‘background-attachment’ after substituting variables. Generated value was ‘auto’.  Falling back to ‘initial’.
Source File: https://cdn.sstatic.net/Sites/stackoverflow/primary.css?v=ee6784011c19
Line: 1
Source Code:
auto

Timestamp: 12/8/2021, 4:43:55 PM
Warning: Property contained reference to invalid variable.  Error in parsing value for ‘color’.  Falling back to ‘inherit’.
Source File: https://cdn.sstatic.net/Sites/stackoverflow/primary.css?v=ee6784011c19
Line: 1
Source Code:
var(--theme-body-font-color)

Timestamp: 12/8/2021, 4:43:55 PM
Warning: Property contained reference to invalid variable.  Error in parsing value for ‘font-family’.  Falling back to ‘inherit’.
Source File: https://cdn.sstatic.net/Sites/stackoverflow/primary.css?v=ee6784011c19
Line: 1
Source Code:
var(--theme-body-font-family)

Timestamp: 12/8/2021, 4:43:52 PM
Warning: Unknown pseudo-class or pseudo-element ‘-webkit-scrollbar-corner’.  Ruleset ignored due to bad selector.
Source File: https://cdn.sstatic.net/Shared/Channels/channels.css?v=47a5e0f03c81
Line: 1, Column: 33550
Source Code: **Omitted for brevity**

Timestamp: 12/8/2021, 4:43:52 PM
Warning: Unknown pseudo-class or pseudo-element ‘-webkit-scrollbar-thumb’.  Ruleset ignored due to bad selector.
Source File: https://cdn.sstatic.net/Shared/Channels/channels.css?v=47a5e0f03c81
Line: 1, Column: 33430
Source Code:  **Omitted for brevity**

Timestamp: 12/8/2021, 4:43:52 PM
Warning: Unknown pseudo-class or pseudo-element ‘-webkit-scrollbar-track’.  Ruleset ignored due to bad selector.
Source File: https://cdn.sstatic.net/Shared/Channels/channels.css?v=47a5e0f03c81
Line: 1, Column: 33315
Source Code: **Omitted for brevity**

Update #2

Seems that SeaMonkey's JavaScript engine is old enough to lack support for optional chaining. Found a similar sounding issue here for another product: Graphs are broken on Seamonkey #718

Answer 1

Seamonkey never was supported as it's not a major browser. You were fortunate that it worked for as long as it did, though.