The logs
```
  Stack:
    Error: Protocol error (Target.createTarget): can't access property "closing", selectedTab is null _setPositionalAttributes@chrome://browser/content/tabbrowser-tabs.js:1123:11
    _insertTabAtIndex@chrome://browser/content/tabbrowser.js:3286:25
    addTab@chrome://browser/content/tabbrowser.js:2710:16
    addTab@chrome://remote/content/shared/TabManager.jsm:24:31
    createTarget@chrome://remote/content/cdp/domains/parent/Target.jsm:90:28
    execute@chrome://remote/content/cdp/domains/DomainCache.jsm:101:25
    execute@chrome://remote/content/cdp/sessions/Session.jsm:64:25
    onPacket@chrome://remote/content/cdp/CDPConnection.jsm:248:36
    onMessage@chrome://remote/content/server/WebSocketTransport.jsm:89:18
    handleEvent@chrome://remote/content/server/WebSocketTransport.jsm:71:14
    openPromptSync@resource://gre/modules/Prompter.jsm:1057:17
    alert@resource://gre/modules/Prompter.jsm:1329:17
    alert@resource://gre/modules/Prompter.jsm:80:7
    _displayPrintingError@chrome://global/content/printUtils.js:572:21
```
My feeling is that something is wrong with the printing test and an alert is generated which blocks following attempts to get the current selected tab (see https://searchfox.org/mozilla-central/source/toolkit/components/printing/content/printUtils.js#572).
["#\\31 51R", 4],
            ["#\\37 9R", 5],
          ]) {
            const prev = await page.$eval("#\\31 71R", el => el.value);
            await page.type(id, val.toString(), { delay: 100 });
            await page.keyboard.press("Tab");
            await page.waitForFunction(
              _prev =>
                getComputedStyle(document.querySelector("#\\31 71R")).value !==
                _prev,
              {},
              prev
            );
            sum += val;
            const total = await page.$eval("#\\31 71R", el => el.value);
            expect(total).withContext(`In ${browserName}`).toEqual(`£${sum}`);
          }
          /
          /
          await page.evaluate(() => {
            window.document
              .querySelectorAll('[data-page-number="4"][class="page"]')[0]
              .scrollIntoView();
          });
          await page.waitForSelector("#\\32 99R", {
            timeout: 0,
          });
          const total = await page.$eval("#\\32 99R", el => el.value);
          expect(total).withContext(`In ${browserName}`).toEqual(`£${sum}`);
        })