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).
await Promise.all(
        pages.map(async ([browserName, page]) => {
          await page.type("#\\34 48R", "61803", { delay: 200 });
          await page.click("#\\34 19R");
          let text = await page.$eval("#\\34 48R", el => el.value);
          expect(text).withContext(`In ${browserName}`).toEqual("61.803,00");
          await page.click("#\\34 48R");
          text = await page.$eval("#\\34 48R", el => el.value);
          expect(text).withContext(`In ${browserName}`).toEqual("61803");
          
          await clearInput(page, "#\\34 48R");
          await page.type("#\\34 48R", "1.61803", { delay: 200 });
          await page.click("#\\34 19R");
          text = await page.$eval("#\\34 48R", el => el.value);
          expect(text).withContext(`In ${browserName}`).toEqual("1,62");
        })
      );
    });
    it("must format the field with 2 digits and leave field with a TAB", async () => {
      await Promise.all(
        pages.map(async ([browserName, page]) => {
          await page.type("#\\34 22R", "2.7182818", { delay: 200 });
          await page.keyboard.press("Tab");
          const text = await page.$eval("#\\34 22R", el => el.value);
          expect(text).withContext(`In ${browserName}`).toEqual("2,72");
          const sum = await page.$eval("#\\34 27R", el => el.value);
          expect(sum).withContext(`In ${browserName}`).toEqual("5,86");
        })
      );
    });