... does "weak encryption" have anything to do with whether the browser trust the certificate? Has it got anything to do with the signature algorithm or signature hash algorithm?
The weak encryption has nothing to do with the certificate:
The hash algorithm used for the certificates signature is SHA-256 which is perfectly fine. The signature is also done with a RSA key of 2048 bit (can be seen in the intermediate CA, which is not shown by you) which is also perfectly fine.
Instead like said in the other answer the weak encryption refers to the use of TLS 1.0 with a CBC cipher. Note that unlike Chrome Firefox does not use RSA key exchange with this site but instead DHE (the cipher is
TLS_DHE_RSA_WITH_AES_128_CBC_SHA), so the information regarding the weak key exchange in the other answer does not apply to Firefox.
To determine what exactly Firefox considers weak I was trying the same cipher with the same length of the DH parameters (1024 bit) as used by the original server but changed the TLS version. It looks like with TLS 1.2 Firefox 70.0.1 considers the encryption strong and only when disabling TLS 1.2 it considers the encryption weak since it now has to use TLS 1.1 or TLS 1.0.