bug(web): Modifier combinations (Ctrl+Alt, Ctrl+Alt+Shift) don't work consistently in Firefox on Ubuntu

**Describe the bug**
When I try to combine multiple modifiers (such as Ctrl+Alt), the combinations never work. Instead, only the last released modifier appears to stay in effect.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to https://keymanweb.com/#lt,Keyboard_lithuanian
2. In the on-screen keyboard, click Ctrl+Alt+Shift+7
3. Note how "&" appears in the textarea
4. Now try using your keyboard and press Ctrl+Alt+Shift+7
5. Observe how the effective modifiers in the on screen keyboard do not reflect what you have pressed and how the output of your typing (if there is any) differs.

**Expected behavior**
The OSK and the physical keyboard should work in the same fashion, but they don't.

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**KeymanWeb (please complete the following information):**
- OS: Debian Linux, Ubuntu 18.04.2
- Browser: Firefox Nightly, Firefox 66
- KeymanWeb Version: 10.0.103
- URL: https://keymanweb.com/#lt,Keyboard_lithuanian
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**Keyboard**
- Keyboard name: lithuanian
- Keyboard version: 1.0
- Language name: Lithuanian