[Affected versions]
Nightly 76.0a1 (2020-03-31) (64-bit)
Beta 75.0b11 (64-bit)

[Affected platforms]
All platforms

[Steps to reproduce]

1- Launch the Firefox Browser and reach any website.
2- Start RDM (Ctrl+Shift+M)
3- Click on the left top button from the toolbar ("Responsive" option)
4- Notice that the device list is shown.
5- Open a new tab (Ctrl + T)
6- Start RDM (Ctrl+Shift+M)
7- Click on the left top button from the toolbar ("Responsive" option)

[Expected result]
device list is shown.

[Actual result]
Device list is not opened.

pushlog_url: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=bbe8633ac35c298c1aabe2fd2b04e6784ab12e67&tochange=901b3eec4eb6757da5558b3bd80eb8aa61eb61bc

2020-03-13T17:50:03: INFO : Narrowed inbound regression window from [bbe8633a, 2a3597aa] (3 builds) to [4dfd98d3, 2a3597aa] (2 builds) (~1 steps left)
2020-03-13T17:50:03: DEBUG : Starting merge handling...
2020-03-13T17:50:03: DEBUG : Using url: https://hg.mozilla.org/integration/autoland/json-pushes?changeset=2a3597aa8d9e6fc158b02b93f26dd1355a955f47&full=1
2020-03-13T17:50:05: DEBUG : Found commit message:
Bug 1604726 - Use a ResizeObserver to responsively size the toolbar. r=bgrins
The RDM toolbar currently scrolls with the page when there is overflow in .browserStack. This is because the browser viewport can be set to a size larger than browser window. The solution here sets makes the toolbar position: fixed and utilizes the ResizeObserver to adjust the size of the device modal/selector to take up the size of the browser stack.
Differential Revision: https://phabricator.services.mozilla.com/D63245
2020-03-13T17:50:05: DEBUG : Did not find a branch, checking all integration branches
2020-03-13T17:50:05: INFO : The bisection is done.
2020-03-13T17:50:05: INFO : Stopped

app_name: firefox
build_date: 2020-02-26 17:12:54.867000
build_file: C:\Users\Clara Guerrero.mozilla\mozregression\persist\901b3eec4eb6-shippable--autoland--target.zip
build_type: inbound
build_url: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/DnOz0ftKRJO1qrnWPdhFOw/runs/0/artifacts/public%2Fbuild%2Ftarget.zip
changeset: 901b3eec4eb6757da5558b3bd80eb8aa61eb61bc
pushlog_url: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=bbe8633ac35c298c1aabe2fd2b04e6784ab12e67&tochange=901b3eec4eb6757da5558b3bd80eb8aa61eb61bc
repo_name: autoland
repo_url: https://hg.mozilla.org/integration/autoland
task_id: DnOz0ftKRJO1qrnWPdhFOw

Tracking Fission DevTools bugs for Fission Nightly (M6) milestone

Bugbug thinks this bug is a regression, but please revert this change in case of error.

Thanks for filing! I'll take a look.

Resolving as FIXED now that Bug 1620098 has landed. I have also verified this has been fixed in Firefox Nightly 78.0a1.