[Affected versions]:
Nightly 79.0a1 (2020-06-23)

Windows 10 (only flash plugin issue)
MacOs 10.14 ( both recent updates and flash plugin issue)
ubuntu 18.04

[Preconditions]
Have devtools.responsive.browserUI.enabled = true

Steps for recent updates issue:

1- Launch the Firefox browser and reach about:addons
2- Hit Ctrl+Shift+M in order to start Responsive Design Mode.
3- Click gear button from "manage your plugins" (present in every subsection of about:addons)
4- Click view recent updates option.
5- After being sent to the new option that appears at the bottom of the list, go back to extensions or recommendations.

Expected Results :
buttons should be displayed correctly with RDM enabled or disabled

Actual Results :
buttons are missing its content

Steps for flash plugin issue

1- Launch the Firefox browser and reach about:addons
2- Hit Ctrl+Shift+M in order to start Responsive Design Mode.
3- Click on plugins
4- Click on 3 dots to trigger dropdown
5- Select "preferences/options" from the list
6- go back to extensions or recommendations.

Expected Results :
buttons should be displayed correctly with RDM enabled or disabled

Actual Results :
buttons are missing its content

note: this does not happen when following steps with RDM closed.

Hi, i'm not able to reproduce this issue in beta 79.0b5 (64-bit).

Best regards,
Clara