When try to input Japanese text using IME, the first character becomes Latin character.

[alice0775]

https://bugzilla.mozilla.org/show_bug.cgi?id=1524043

Unable type Japanese text using IME.
Unable to search with Japanese text.

Reproducible: always

Steps To Reproduce:

1. Open https://support.mozilla.org/ja/products/firefox?as=u&utm_source=inproduct
2. Focus search field in the page
3. Turn IME on
4. Try スペル (press key s u p e r u)

Actual Results:
When first key(s) is pressed, the page will automatically transition to https://support.mozilla.org/ja/products/firefox?as=u&utm_source=inproduct
So,now, sうぺる in the search field, s is committed, the others are under construction.

Expected Results:
すぺる in the search field, and all characters are under construction.

[Phroneris]

Same.
I tried to search with "言語（gen-go)", but it was pain.

Expected usage:

0. Focus on the search bar
1. I type G, E, N, G and O, so I get "げんご" in hiragana staying suspended for kanji conversion
2. I press Henkan to convert my hiragana to kanji "言語"
3. I press Enter to determine the conversion
4. Search starts
5. Search result shows something about "言語"

Actual usage:

0. Focus on the search bar
1. I type G, E, ... (quickly)
2. My hiragana is determined automatically as "げ"
3. Search starts anyway
4. I'm continuing typing N, G and O, so I get determined "げ" and following suspended "んご"
5. I press Henkan then only the latter "んご" is converted as "ンゴ" (for example)
6. Search restarts
7. Search result shows something about "げンゴ"