I've confirmed that Bug 1092007 was indeed the underlying issue here, and it's now fixed in Nightly. Here's a screenshot of dbaron's partially-simplified-testcase, viewed side by side in Firefox 68.0.2 (overlapping content) vs. in Nightly (no overlap). Per https://github.com/webcompat/web-bugs/issues/30754#issuecomment-520489948 , LinkedIn was kind enough to change their markup to work around this issue, so this is no longer reproducible on the live site. But the testcase allowed me to double check that we have now actually fixed it on our end, too.
Bug 1559917 Comment 14 Edit History
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I've confirmed that Bug 1092007 was indeed the underlying issue here, and it's now fixed in Nightly. Here's a screenshot of dbaron's partially-simplified-testcase, viewed side by side in Firefox 68.0.2 (overlapping content) vs. in Nightly (no overlap). Per https://github.com/webcompat/web-bugs/issues/30754#issuecomment-520489948 , LinkedIn was kind enough to change their markup to work around this issue, so this should no longer be reproducible on the live site even in the current release of Firefox. But the testcase allowed me to double check that we have now actually fixed the underlying original issue on our end (in Nightly), too.