Document support for Firefox ESR

[JasonWeill]

Problem

We've seen issues, most recently jupyterlab/jupyterlab#16391, from users trying to use newer versions of JupyterLab with Firefox ESR (extended support release). Firefox ESR typically gets "major" updates about every 42 weeks (about 10 months) and minor updates about every 4 weeks. Some organizations choose the ESR channel for internal Firefox users to reduce the operational load caused by frequent mainline browser updates. This can cause problems because CSS and JavaScript support in Firefox ESR can lag significantly behind mainline Firefox releases.

Our documentation only describes support for "the latest versions", implying mainline release channels, of several popular browsers:

https://github.com/jupyterlab/jupyterlab/blob/2c718c1be098f11d602a7b82b534a8654419533f/docs/source/getting_started/installation.rst?plain=1#L133-L143

We do not have any documented recommendation for users on ESR browsers.

Proposed Solution

Modify documentation for Jupyter front-end projects (JupyterLab and Jupyter Notebook) that we support the latest versions of the mainline Firefox release channel. ESR users may submit pull requests for consideration to fix bugs that affect Firefox ESR, as long as these don't create undue regressions for users of the newest mainline version. Firefox ESR users may avoid breakages introduced in newer versions by using older but still supported Jupyter versions, such as (as of 2024-06-05) JupyterLab 4.0.x or 4.1.x.

Additional context

Discussed at the 2024-06-05 Jupyter frontends call.

See also #239, a proposal to create an official long-term support (LTS) release for JupyterLab, which could be intended for use with Firefox ESR.