User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0

Steps to reproduce:

Tried to access my Google drive on the web

Actual results:

quite unexpectedly Google Drive became unreachable though quite recently it was.
Says:
You are currently using Firefox 60, for which an update is highly recommended!
Update your browser to use Google Drive, Docs, Sheets, Sites, Slides, and Forms
To use Google Drive, Docs, Sheets, Sites, Slides, and Forms, you need to use a supported web browser.

REPRODUCIBLE with installation of official De SeaMonkey 2.53.9 Mozilla/5.0 (NT 10.0; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0 Build 20210808162125 (Newly created User Profile, Default Classic Theme) on German WIN7 64bit

STR:

1. Visit https://www.google.com/
2. Select service "Google Drive"
   » expected: https://drive.google.com/drive/my-drive
   actual: https://support.google.com/drive/answer/6283888?p=unsupported_browser&visit_id=637650309569707525-3921651315&rd=1
   Link to reach Google Drive in top right corner of the web page will not work 😥

Until now that is a Google problem. They do not want to see SM on their page.

But yo can reach it with a trick:
11. With add on "User Agent Switcher" pretend to use "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:90.0a1) Gecko/20100101 Firefox/90.0a1"
12. Redo from (1)
» In step 2 you will reach https://drive.google.com/drive/my-drive

But there will remain real problema. An example:
I tried to reach an existing spreadsheet.
In SM double click on the Thumbnail preview does nothing - at least it will not open the spreadsheet. In Chromium Spreadsheet will be opened.
But with ˋrigt click → open → With Google Sheetsˊ I can open and edit the page; I have to drop SM context menu with <Esc> before the Google context menu becomes usable

(In reply to Rainer Bielefeld from comment #1)

REPRODUCIBLE with installation of official De SeaMonkey 2.53.9 Mozilla/5.0 (NT 10.0; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0 Build 20210808162125 (Newly created User Profile, Default Classic Theme) on German WIN7 64bit

STR:

1. Visit https://www.google.com/
2. Select service "Google Drive"
   » expected: https://drive.google.com/drive/my-drive
   actual: https://support.google.com/drive/answer/6283888?p=unsupported_browser&visit_id=637650309569707525-3921651315&rd=1
   Link to reach Google Drive in top right corner of the web page will not work 😥

Until now that is a Google problem. They do not want to see SM on their page.

But yo can reach it with a trick:
11. With add on "User Agent Switcher" pretend to use "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:90.0a1) Gecko/20100101 Firefox/90.0a1"
12. Redo from (1)
» In step 2 you will reach https://drive.google.com/drive/my-drive

But there will remain real problema. An example:
I tried to reach an existing spreadsheet.
In SM double click on the Thumbnail preview does nothing - at least it will not open the spreadsheet. In Chromium Spreadsheet will be opened.
But with ˋrigt click → open → With Google Sheetsˊ I can open and edit the page; I have to drop SM context menu with <Esc> before the Google context menu becomes usable
While replying to your comment coudl not add a snapshot.
Please teach.

OK! Should I Install 2.59 branch?
Is it secure?
Will it inherit all my strange settings (including paths to file locations, mail tags, mail filters etc.) automatically?
Thank you.

Please ignore previous comments from #2 through #4.
(I don't know how to delete them)

User Agent Switcher could not be installed because it is incompatiadle with SeaMonkey 2.53.9 too!!

Confirmed on Linux 64 bit
I thìnk the problem is not related to Google do not want to see SM, but to the fact the gecko engine is still the one of Firefox 60

My current user agent string is: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Firefox/60.0 SeaMonkey/2.53.9.1

I just made a try going to:
about:config

• right click somewhere
• New, String
• as preference name use: general.useragent.override.drive.google.com
• as enter string value use: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/60.0 SeaMonkey/2.53.9.1
  so I changed only "rv:60.0" to "rv:68.0" and now Drive work

This is only a work around, please update the Seamonkey Gecko engine