NVDA reads "Not selected" for table columns when tabindex is specified with Firefox

[Ajaykr]

NVDA reads "Not selected" for table columns when tabindex is specified with Firefox browser.
test_Simpletable.txt

Steps to reproduce:

1. Change the extension of attached file to html.
2. Open the html file using Firefox.
3. Open NVDA
4. Try to read the content of rendered html output in focus mode.
5. Observe the output of NVDA for table columns with tabindex property specified.

Actual behavior:

The output is like below:
"Person column header not selected row 1 column 1"
The additional "not selected" is being read out.

Expected behavior:

The output should have been:
"Person column header row 1 column 1"

System configuration

System Type: x64-based PC
Processor: Intel(R) Core(TM) i7-9850H CPU @ 2.60GHz, 2592 Mhz, 6 Core(s), 12 Logical Processor(s)
Installed Physical Memory (RAM): 32.0 GB

NVDA installed/portable/running from source:

Installed

NVDA version:

NVDA Version: 2021.2

Windows version:

Microsoft Windows 10 Enterprise
Version: 10.0.18363 Build 18363

Name and version of other software in use when reproducing the issue:

Firefox Version : 93.0 (64-bit)

Other information about your system:

Other questions

Does the issue still occur after restarting your computer?

Yes, the issue still occurs after restarting the computer.

Have you tried any other versions of NVDA? If so, please report their behaviors.

No

If NVDA add-ons are disabled, is your problem still occurring?

Not applicable as no add-ons installed.

Does the issue still occur after you run the COM Registration Fixing Tool in NVDA's tools menu?

Not tried.

[lukaszgo1]

cc @jcsteh This is especially annoying when viewing messages on messenger.com

[lukaszgo1]

Just FYI - this does not occur with JAWS.

[Adriani90]

@jcsteh should we report this to Bugzilla? Or is this something which needs to be solved in NVDA?

cc: @SaschaCowley

[Adriani90]

Test case:
code sample: https://github.com/BabyElias/kolibri-design-system/blob/gsoc-table/lib/KTable/index.vue
and the tables can be viewed at this link: https://deploy-preview-669--kolibri-design-system.netlify.app/playground/