Tested with:
Nightly 91.0a1 (2021-06-24)

Tested on:
Win 10

Preconditions:
In about:config, set pdfjs.enableXfa = true

Steps:

1. Launch Firefox
2. Open the attached pdf.

Actual result:
Text on the button is cut off.

Expected result:
Text should be properly displayed on the bottom.

The latest Nightly v91.0a1 from 2021-06-27 does not even show the buttons that reproduced the issue anymore.

Calixte, can you check if this is fixed by the line height patch?

The buttons reappeared after the fixes pushed in bug 1719380:

2021-07-09T01:50:01.425000: DEBUG : Found commit message:
Bug 1719380 - Update pdf.js to version 2.10.263 r=pdfjs-reviewers,marco

Differential Revision: https://phabricator.services.mozilla.com/D119237

2021-07-09T01:50:01.425000: DEBUG : Did not find a branch, checking all integration branches
2021-07-09T01:50:01.427000: INFO : The bisection is done.
2021-07-09T01:50:01.428000: INFO : Stopped

The issue now occurs as originally reported.

Thanks, so this is not fixed yet.

Same behaviour on the following pdfs attached.

This is fixed. I'm not sure what fixed it (maybe the latest release from bug 1722768?).

Calixte, do you want to add one or more of these PDFs to the reftests? Or is this already covered by some existing one?

This issue appears to have been fixed by:
Bug 1720361 - Update pdf.js to version 2.10.316 r=pdfjs-reviewers,calixte
Differential Revision: https://phabricator.services.mozilla.com/D119800

I think we've enough tests for that stuff.