`css/css-transitions/CSSTransition-startTime.tentative.html` is not stable in Firefox

[Loirooriol]

See https://github.com/web-platform-tests/wpt/pull/51880/checks?check_run_id=40119812617

 1:05.40 INFO ### /css/css-transitions/CSSTransition-startTime.tentative.html ###
 1:05.40 INFO |                               Subtest                                |          Results           |                                                  Messages                                                 |
 1:05.40 INFO |----------------------------------------------------------------------|----------------------------|-----------------------------------------------------------------------------------------------------------|
 1:05.40 INFO |                                                                      | OK                         |                                                                                                           |
 1:05.40 INFO | `The start time of a newly-created transition is unresolved`         | PASS                       |                                                                                                           |
 1:05.40 INFO | `The start time of transitions is based on when they are generated`  | PASS                       |                                                                                                           |
 1:05.40 INFO | `The start time of a transition can be set`                          | **FAIL: 1/10, PASS: 9/10** | `assert_approx_equals: Check setting of startTime actually works expected 44.34 +/- 0.0005 but got 44.32` |
 1:05.40 INFO | `The start time can be set to seek a transition`                     | PASS                       |                                                                                                           |
 1:05.40 INFO | `Seeking a transition using start time dispatches transition events` | PASS                       |                                                                                                           |
 1:05.40 INFO 
 1:05.40 INFO ## Unstable results ##

This prevents landing PRs that want to add something into css/css-transitions/support/helper.js, see #35978

[Loirooriol]

Disabled the problematic subtest and filed https://bugzilla.mozilla.org/show_bug.cgi?id=1959231

[gsnedders]

We shouldn't be disabling tests for all browsers just because they happen to be flaky in one; unless changes are super-frequent to those tests, relying on admins to merge is typically not-overly-burdensome.